Linkage map of Salmonella typhimurium, Edition VI

Regulation of Flagellum Biosynthesis in Response to Cell Envelope Stress in Salmonella enterica Serovar Typhimurium

Flagellum-driven motility of Salmonella enterica serovar Typhimurium facilitates host colonization. However, the large extracellular flagellum is also a prime target for the immune system. As consequence, expression of flagella is bistable within a population of Salmonella, resulting in flagellated and nonflagellated subpopulations. This allows the bacteria to maximize fitness in hostile environments. The degenerate EAL domain protein RflP (formerly YdiV) is responsible for the bistable expression of flagella by directing the flagellar master regulatory complex FlhD₄C₂ with respect to proteolytic degradation. Information concerning the environmental cues controlling expression of rflP and thus about the bistable flagellar biosynthesis remains ambiguous. Here, we demonstrated that RflP responds to cell envelope stress and alterations of outer membrane integrity. Lipopolysaccharide (LPS) truncation mutants of Salmonella Typhimurium exhibited increasing motility defects due to downregulation of flagellar gene expression. Transposon mutagenesis and genetic profiling revealed that σ²⁴ (RpoE) and Rcs phosphorelay-dependent cell envelope stress response systems sense modifications of the lipopolysaccaride, low pH, and activity of the complement system. This subsequently results in activation of RflP expression and degradation of FlhD₄C₂ via ClpXP. We speculate that the presence of diverse hostile environments inside the host might result in cell envelope damage and would thus trigger the repression of resource-costly and immunogenic flagellum biosynthesis via activation of the cell envelope stress response.

Pathogenic bacteria such as Salmonella Typhimurium sense and adapt to a multitude of changing and stressful environments during host infection. At the initial stage of gastrointestinal colonization, Salmonella uses flagellum-mediated motility to reach preferred sites of infection. However, the flagellum also constitutes a prime target for the host’s immune response. Accordingly, the pathogen needs to determine the spatiotemporal stage of infection and control flagellar biosynthesis in a robust manner. We found that Salmonella uses signals from cell envelope stress-sensing systems to turn off production of flagella. We speculate that downregulation of flagellum synthesis after cell envelope damage in hostile environments aids survival of Salmonella during late stages of infection and provides a means to escape recognition by the immune system.

Variability in bacterial flagella re-growth patterns after breakage

Many bacteria swim through liquids or crawl on surfaces by rotating long appendages called flagella. Flagellar filaments are assembled from thousands of subunits that are exported through a narrow secretion channel and polymerize beneath a capping scaffold at the tip of the growing filament. The assembly of a flagellum uses a significant proportion of the biosynthetic capacities of the cell with each filament constituting ~1% of the total cell protein. Here, we addressed a significant question whether a flagellar filament can form a new cap and resume growth after breakage. Re-growth of broken filaments was visualized using sequential 3-color fluorescent labeling of filaments after mechanical shearing. Differential electron microscopy revealed the formation of new cap structures on broken filaments that re-grew. Flagellar filaments are therefore able to re-grow if broken by mechanical shearing forces, which are expected to occur frequently in nature. In contrast, no re-growth was observed on filaments that had been broken using ultrashort laser pulses, a technique allowing for very local damage to individual filaments. We thus conclude that assembly of a new cap at the tip of a broken filament depends on how the filament was broken.

Case for the genetic code as a triplet of triplets

The efficiency of codon translation in vivo is controlled by many factors, including codon context. At a site early in the Salmonella flgM gene, the effects on translation of replacing codons Thr6 and Pro8 of flgM with synonymous alternates produced a 600-fold range in FlgM activity. Synonymous changes at Thr6 and Leu9 resulted in a twofold range in FlgM activity. The level of FlgM activity produced by any codon arrangement was directly proportional to the degree of in vivo ribosome stalling at synonymous codons. Synonymous codon suppressors that corrected the effect of a translation-defective synonymous flgM allele were restricted to two codons flanking the translation-defective codon. The various codon arrangements had no apparent effects on flgM mRNA stability or predicted mRNA secondary structures. Our data suggest that efficient mRNA translation is determined by a triplet-of-triplet genetic code. That is, the efficiency of translating a particular codon is influenced by the nature of the immediately adjacent flanking codons. A model explains these codon-context effects by suggesting that codon recognition by elongation factor-bound aminoacyl-tRNA is initiated by hydrogen bond interactions between the first two nucleotides of the codon and anticodon and then is stabilized by base-stacking energy over three successive codons.

Characterization of Novel Factors Involved in Swimming and Swarming Motility in Salmonella enterica Serovar Typhimurium

Salmonella enterica utilizes flagellar motility to swim through liquid environments and on surfaces. The biosynthesis of the flagellum is regulated on various levels, including transcriptional and posttranscriptional mechanisms. Here, we investigated the motility phenotype of 24 selected single gene deletions that were previously described to display swimming and swarming motility effects. Mutations in flgE, fliH, ydiV, rfaG, yjcC, STM1267 and STM3363 showed an altered motility phenotype. Deletions of flgE and fliH displayed a non-motile phenotype in both swimming and swarming motility assays as expected. The deletions of STM1267, STM3363, ydiV, rfaG and yjcC were further analyzed in detail for flagellar and fimbrial gene expression and filament formation. A ΔydiV mutant showed increased swimming motility, but a decrease in swarming motility, which coincided with derepression of curli fimbriae. A deletion of yjcC, encoding for an EAL domain-containing protein, increased swimming motility independent on flagellar gene expression. A ΔSTM1267 mutant displayed a hypermotile phenotype on swarm agar plates and was found to have increased numbers of flagella. In contrast, a knockout of STM3363 did also display an increase in swarming motility, but did not alter flagella numbers. Finally, a deletion of the LPS biosynthesis-related protein RfaG reduced swimming and swarming motility, associated with a decrease in transcription from flagellar class II and class III promoters and a lack of flagellar filaments.

Molecular ruler determines needle length for the Salmonella Spi-1 injectisome

The type-III secretion (T3S) systems of bacteria are part of self-assembling nanomachines: the bacterial flagellum that enables cells to propel themselves through liquid and across hydrated surfaces, and the injectisome that delivers pathogenic effector proteins into eukaryotic host cells. Although the flagellum and injectisome serve different purposes, they are evolutionarily related and share many structural similarities. Core features to these T3S systems are intrinsic length control mechanisms for external cellular projections: the hook of the flagellum and the injectisome needle. We present evidence that the Spi-1 injectisome, like the Salmonella flagellar hook, uses a secreted molecular ruler, InvJ, to determine needle length. This result supports a universal length control mechanism using molecular rulers for T3S systems.

ATPase-independent type-III protein secretion in Salmonella enterica

Type-III protein secretion systems are utilized by gram-negative pathogens to secrete building blocks of the bacterial flagellum, virulence effectors from the cytoplasm into host cells, and structural subunits of the needle complex. The flagellar type-III secretion apparatus utilizes both the energy of the proton motive force and ATP hydrolysis to energize substrate unfolding and translocation. We report formation of functional flagella in the absence of type-III ATPase activity by mutations that increased the proton motive force and flagellar substrate levels. We additionally show that increased proton motive force bypassed the requirement of the Salmonella pathogenicity island 1 virulence-associated type-III ATPase for secretion. Our data support a role for type-III ATPases in enhancing secretion efficiency under limited secretion substrate concentrations and reveal the dispensability of ATPase activity in the type-III protein export process.

Comparative analysis of the secretion capability of early and late flagellar type III secretion substrates

A remarkable feature of the flagellar-specific type III secretion system (T3SS) is the selective recognition of a few substrate proteins among the many thousand cytoplasmic proteins. Secretion substrates are divided into two specificity classes: early substrates secreted for hook-basal body (HBB) construction and late substrates secreted after HBB completion. Secretion was reported to require a disordered N-terminal secretion signal, mRNA secretion signals within the 5'-untranslated region (5'-UTR) and for late substrates, piloting proteins known as the T3S chaperones. Here, we utilized translational β-lactamase fusions to probe the secretion efficacy of the N-terminal secretion signal of fourteen secreted flagellar substrates in Salmonella enterica. We observed a surprising variety in secretion capability between flagellar proteins of the same secretory class. The peptide secretion signals of the early-type substrates FlgD, FlgF, FlgE and the late-type substrate FlgL were analysed in detail. Analysing the role of the 5'-UTR in secretion of flgB and flgE revealed that the native 5'-UTR substantially enhanced protein translation and secretion. Based on our data, we propose a multicomponent signal that drives secretion via the flagellar T3SS. Both mRNA and peptide signals are recognized by the export apparatus and together with substrate-specific chaperones allowing for targeted secretion of flagellar substrates.

The effects of codon context on in vivo translation speed

We developed a bacterial genetic system based on translation of the his operon leader peptide gene to determine the relative speed at which the ribosome reads single or multiple codons in vivo. Low frequency effects of so-called "silent" codon changes and codon neighbor (context) effects could be measured using this assay. An advantage of this system is that translation speed is unaffected by the primary sequence of the His leader peptide. We show that the apparent speed at which ribosomes translate synonymous codons can vary substantially even for synonymous codons read by the same tRNA species. Assaying translation through codon pairs for the 5'- and 3'- side positioning of the 64 codons relative to a specific codon revealed that the codon-pair orientation significantly affected in vivo translation speed. Codon pairs with rare arginine codons and successive proline codons were among the slowest codon pairs translated in vivo. This system allowed us to determine the effects of different factors on in vivo translation speed including Shine-Dalgarno sequence, rate of dipeptide bond formation, codon context, and charged tRNA levels.

The Salmonella Spi1 virulence regulatory protein HilD directly activates transcription of the flagellar master operon flhDC

Infection of intestinal epithelial cells is dependent on the Salmonella enterica serovar Typhimurium pathogenicity island 1 (Spi1)-encoded type III injectisome system and flagellar motility. Thus, the expression of virulence and flagellar genes is subject to tight regulatory control mechanisms in order to ensure the correct spatiotemporal production of the respective gene products. In this work, we reveal a new level of cross-regulation between the Spi1 and flagellar regulatory systems. Transposon mutagenesis identified a class of mutants that prevented flhDC autorepression by overexpressing HilD. HilD, HilC, RtsA, and HilA comprise a positive regulatory circuit for the expression of the Spi1 genes. Here, we report a novel transcriptional cross talk between the Spi1 and flagellar regulons where HilD transcriptionally activates flhDC gene expression by binding to nucleotides -68 to -24 upstream from the P5 transcriptional start site. We additionally show that, in contrast to the results of a previous report, HilA does not affect flagellar gene expression. Finally, we discuss a model of the cross-regulation network between Spi1 and the flagellar system and propose a regulatory mechanism via the Spi1 master regulator HilD that would prime flagellar genes for rapid reactivation during host infection.

RflM functions as a transcriptional repressor in the autogenous control of the Salmonella Flagellar master operon flhDC

Motility of bacteria like Salmonella enterica is a highly regulated process that responds to a variety of internal and external stimuli. A hierarchy of three promoter classes characterizes the Salmonella flagellar system, and the onset of flagellar gene expression depends on the oligomeric regulatory complex and class 1 gene product FlhD(4)C(2). The flhDC promoter is a target for a broad range of transcriptional regulators that bind within the flhDC promoter region and either negatively or positively regulate flhDC operon transcription. In this work, we demonstrate that the RflM protein is a key component of flhDC regulation. Transposon mutagenesis was performed to investigate a previously described autoinhibitory effect of the flagellar master regulatory complex FlhD(4)C(2). RflM is a LuxR homolog that functions as a flagellar class 1 transcriptional repressor. RflM was found to be the negative regulator of flhDC expression that is responsible for the formerly described autoinhibitory effect of the FlhD(4)C(2) complex on flhDC operon transcription (K. Kutsukake, Mol. Gen. Genet. 254:440-448, 1997). We conclude that upon commencement of flagellar gene expression, the FlhD(4)C(2) complex initiates a regulatory feedback loop by activating rflM gene expression. rflM encodes a transcriptional repressor, RflM, which fine-tunes flhDC expression levels.

Selective purification of recombinant neuroactive peptides using the flagellar type III secretion system

The structure, assembly, and function of the bacterial flagellum involves about 60 different proteins, many of which are selectively secreted via a specific type III secretion system (T3SS) (J. Frye et al., J. Bacteriol. 188:2233–2243, 2006). The T3SS is reported to secrete proteins at rates of up to 10,000 amino acid residues per second. In this work, we showed that the flagellar T3SS of Salmonella enterica serovar Typhimurium could be manipulated to export recombinant nonflagellar proteins through the flagellum and into the surrounding medium. We translationally fused various neuroactive peptides and proteins from snails, spiders, snakes, sea anemone, and bacteria to the flagellar secretion substrate FlgM. We found that all tested peptides of various sizes were secreted via the bacterial flagellar T3SS. We subsequently purified the recombinant μ-conotoxin SIIIA (rSIIIA) from Conus striatus by affinity chromatography and confirmed that T3SS-derived rSIIIA inhibited mammalian voltage-gated sodium channel Na_V1.2 comparably to chemically synthesized SIIIA.

Manipulation of the flagellar secretion system bypasses the problems of inclusion body formation and cellular degradation that occur during conventional recombinant protein expression. This work serves as a proof of principle for the use of engineered bacterial cells for rapid purification of recombinant neuroactive peptides and proteins by exploiting secretion via the well-characterized flagellar type III secretion system.

A constitutively mannose-sensitive agglutinating Salmonella enterica subsp. enterica serovar typhimurium strain, carrying a transposon in the fimbrial usher gene stbC, exhibits multidrug resistance and flagellated phenotypes

Static broth culture favors Salmonella enterica subsp. enterica serovar Typhimurium to produce type 1 fimbriae, while solid agar inhibits its expression. A transposon inserted in stbC, which would encode an usher for Stb fimbriae of a non-flagellar Salmonella enterica subsp. enterica serovar Typhimurium LB5010 strain, conferred it to agglutinate yeast cells on both cultures. RT-PCR revealed that the expression of the fimbrial subunit gene fimA, and fimZ, a regulatory gene of fimA, were both increased in the stbC mutant when grown on LB agar; fimW, a repressor gene of fimA, exhibited lower expression. Flagella were observed in the stbC mutant and this phenotype was correlated with the motile phenotype. Microarray data and RT-PCR indicated that the expression of three genes, motA, motB, and cheM, was enhanced in the stbC mutant. The stbC mutant was resistant to several antibiotics, consistent with the finding that expression of yhcQ and ramA was enhanced. A complementation test revealed that transforming a recombinant plasmid possessing the stbC restored the mannose-sensitive agglutination phenotype to the stbC mutant much as that in the parental Salmonella enterica subsp. enterica serovar Typhimurium LB5010 strain, indicating the possibility of an interplay of different fimbrial systems in coordinating their expression.

The role of the FliK molecular ruler in hook-length control in Salmonella enterica

A molecular ruler, FliK, controls the length of the flagellar hook. FliK measures hook length and catalyses the secretion-substrate specificity switch from rod-hook substrate specificity to late substrate secretion, which includes the filament subunits. Here, we show normal hook-length control and filament assembly in the complete absence of the C-ring thus refuting the previous 'cup' model for hook-length control. Mutants of C-ring components, which are reported to produce short hooks, show a reduced rate of hook-basal body assembly thereby allowing for a premature secretion-substrate specificity switch. Unlike fliK null mutants, hook-length control in an autocleavage-defective mutant of flhB, the protein responsible for the switch to late substrate secretion, is completely abolished. FliK deletion variants that retain the ability to measure hook length are secreted thus demonstrating that FliK directly measures rod-hook length during the secretion process. Finally, we present a unifying model accounting for all published data on hook-length control in which FliK acts as a molecular ruler that takes measurements of rod-hook length while being intermittently secreted during the assembly process of the hook-basal body complex.

C-ring requirement in flagellar type III secretion is bypassed by FlhDC upregulation

The cytoplasmic C-ring of the flagellum consists of FliG, FliM and FliN and acts as an affinity cup to localize secretion substrates for protein translocation via the flagellar-specific type III secretion system. Random T-POP transposon mutagenesis was employed to screen for insertion mutants that allowed flagellar type III secretion in the absence of the C-ring using the flagellar type III secretion system-specific hook-beta-lactamase reporter (Lee and Hughes, 2006). Any condition resulting in at least a twofold increase in flhDC expression was sufficient to overcome the requirement for the C-ring and the ATPase complex FliHIJ in flagellar type III secretion. Insertions in known and unknown flagellar regulatory loci were isolated as well as chromosomal duplications of the flhDC region. The twofold increased flhDC mRNA level coincided in a twofold increase in the number of hook-basal bodies per cell as analysed by fluorescent microscopy. These results indicate that the C-ring functions as a nonessential affinity cup-like structure during flagellar type III secretion to enhance the specificity and efficiency of the secretion process.

Characterization of Escherichia coli DNA lesions generated within J774 macrophages

Macrophages are armed with multiple oxygen-dependent and -independent bactericidal properties. However, the respiratory burst, generating reactive oxygen species, is believed to be a major cause of bacterial killing. We exploited the susceptibility of Escherichia coli in macrophages to characterize the effects of the respiratory burst on intracellular bacteria. We show that E. coli strains recovered from J774 macrophages exhibit high rates of mutations. We report that the DNA damage generated inside macrophages includes DNA strand breaks and the modification 8-oxo-2'-deoxyguanosine, which are typical oxidative lesions. Interestingly, we found that under these conditions, early in the infection the majority of E. coli cells are viable but gene expression is inhibited. Our findings demonstrate that macrophages can cause severe DNA damage to intracellular bacteria. Our results also suggest that protection against the macrophage-induced DNA damage is an important component of the bacterial defense mechanism within macrophages.

Fusidic acid-resistant EF-G perturbs the accumulation of ppGpp

Reductions in growth rate caused by fusidic acid-resistant EF-G mutants in Salmonella typhimurium correlate strongly with increased mean cell size. This is unusual because growth rate and cell size normally correlate positively. The global transcription regulator molecule ppGpp has a role in co-ordinating growth rate and division, and its basal level normally correlates inversely with cell size at division. We show that fusidic acid-resistant EF-G mutants have perturbed ppGpp basal levels during steady-state growth and perturbed induced levels during starvation. One mutation, fusA1, associated with the slowest growth rate and largest cell size, causes a reduction in the basal level of ppGpp to one-third of that found in the wild-type strain. Other fusA mutants with intermediate or wild-type growth rates and cell sizes have either normal or increased basal levels of ppGpp. There is an inverse relationship between the basal level of ppGpp in vivo and the degree to which translation dependent on mutant EF-G is inhibited by ppGpp in vitro. This enhanced interaction between mutant EF-G and ppGpp correlates with an increased KM for GTP. Our results suggest that mutant EF-G modulates the production of ppGpp by the RelA (PSI) pathway. In conclusion, fusidic acid-resistant EF-G mutations alter the level of ppGpp and break the normal relationship between growth rate and cell size at division. It would not be surprising if other phenotypes associated with these mutants, such as loss of virulence, were also related to perturbations in ppGpp levels effected through altered transcription patterns.

Growth phase-regulated induction of Salmonella-induced macrophage apoptosis correlates with transient expression of SPI-1 genes

Invasive Salmonella has been reported to induce apoptosis in a fraction of infected macrophages within 2 to 14 h from the time of infection by a mechanism involving the type III secretion machinery encoded by the Salmonella pathogenicity island 1 (SPI-1). Here, we show that bacteria in the transition from logarithmic to stationary phase cause 90% of the macrophages to undergo phagocytosis-independent, caspase-mediated apoptosis within 30 to 60 min of infection. The ability of Salmonella to induce this rapid apoptosis was growth phase regulated and cell type restricted, with epithelial cells being resistant. Apoptosis induction was also abrogated by disruption of the hilA gene (encoding a regulator of SPI-1 genes) and by the expression of a constitutively active PhoPQ. hilA itself and a subset of SPI-1 genes were transiently expressed during aerobic growth in liquid medium. Interestingly, however, hilA was found to be required only for the expression of the prgH gene, while sipB, invA, and invF were expressed in a hilA-independent manner. The expression of SPI-1 genes and the secretion of invasion-associated proteins correlated temporally with the induction of apoptosis and are likely to represent its molecular basis. Thus, growth phase transition regulates the expression and secretion of virulence determinants and represents the most efficient environmental cue for apoptosis induction reported to date.

Branched-chain amino acid biosynthesis in Salmonella typhimurium: a quantitative analysis

We report here the first quantitative study of the branched-chain amino acid biosynthetic pathway in Salmonella typhimurium LT2. The intracellular levels of the enzymes of the pathway and of the 2-keto acid intermediates were determined under various physiological conditions and used for estimation of several of the fluxes in the cells. The results led to a revision of previous ideas concerning the way in which multiple acetohydroxy acid synthase (AHAS) isozymes contribute to the fitness of enterobacteria. In wild-type LT2, AHAS isozyme I provides most of the flux to valine, leucine, and pantothenate, while isozyme II provides most of the flux to isoleucine. With acetate as a carbon source, a strain expressing AHAS II only is limited in growth because of the low enzyme activity in the presence of elevated levels of the inhibitor glyoxylate. A strain with AHAS I only is limited during growth on glucose by the low tendency of this enzyme to utilize 2-ketobutyrate as a substrate; isoleucine limitation then leads to elevated threonine deaminase activity and an increased 2-ketobutyrate/2-ketoisovalerate ratio, which in turn interferes with the synthesis of coenzyme A and methionine. The regulation of threonine deaminase is also crucial in this regard. It is conceivable that, because of fundamental limitations on the specificity of enzymes, no single AHAS could possibly be adequate for the varied conditions that enterobacteria successfully encounter.

DNA adenine methylase mutants of Salmonella typhimurium and a novel dam-regulated locus

Mutants of Salmonella typhimurium lacking DNA adenine methylase were isolated; they include insertion and deletion alleles. The dam locus maps at 75 min between cysG and aroB, similar to the Escherichia coli dam gene. Dam- mutants of S. typhimurium resemble those of E coli in the following phenotypes: (1) increased spontaneous mutations, (2) moderate SOS induction, (3) enhancement of duplication segregation, (4) inviability of dam recA and dam recB mutants, and (5) suppression of the inviability of the dam recA and dam recB combinations by mutations that eliminate mismatch repair. However, differences between S. typhimurium and E. coli dam mutants are also found: (1) S. typhimurium dam mutants do not show increased UV sensitivity, suggesting that methyl-directed mismatch repair does not participate in the repair of UV-induced DNA damage in Salmonella. (2) S. typhimurium dam recJ mutants are viable, suggesting that the Salmonella RecJ function does not participate in the repair of DNA strand breaks formed in the absence of Dam methylation. We also describe a genetic screen for detecting novel genes regulated by Dam methylation and a locus repressed by Dam methylation in the S. typhimurium virulence (or "cryptic") plasmid.

An O-antigen processing function for Wzx (RfbX): a promising candidate for O-unit flippase

O antigen is the major cell surface antigen of gram-negative bacteria, and the genes responsible for its synthesis are located in a single gene cluster. The wzx (rbfX) gene, which is characteristic of the major class of O-antigen gene clusters, encodes a hydrophobic protein with 12 potential transmembrane segments. We demonstrate that a wzx mutant accumulates undecaprenol pyrophosphate-linked O units which appear to be on the cytoplasmic side of the cytoplasmic membrane, suggesting that the wzx gene encodes a flippase for O-unit translocation across that membrane.

Role of Erf recombinase in P22-mediated plasmid transduction

In the absence of host RecA function, plasmid transduction by bacteriophage P22 can be mediated by Erf recombinase. Erf is not carried on the infecting particle but synthesized upon infection. In the recipient cell, Erf can promote both generalized plasmid transduction (which requires the circularization of plasmids transduced as linear multimers) and specialized plasmid transduction (which requires the release of plasmid DNA from linear plasmid-phage cointegrates). Both processes of Erf-mediated plasmid transduction require host RecBCD function. In contrast, RecBCD is not required for Erf-mediated circularization of P22 DNA.

Sequence, regulation, and functions of fis in Salmonella typhimurium

The fis operon from Salmonella typhimurium has been cloned and sequenced, and the properties of Fis-deficient and Fis-constitutive strains were examined. The overall fis operon organization in S. typhimurium is the same as that in Escherichia coli, with the deduced Fis amino acid sequences being identical between both species. While the open reading frames upstream of fis have diverged slightly, the promoter regions between the two species are also identical between -49 and +94. Fis protein and mRNA levels fluctuated dramatically during the course of growth in batch cultures, peaking at approximately 40,000 dimers per cell in early exponential phase, and were undetectable after growth in stationary phase. fis autoregulation was less effective in S. typhimurium than that in E. coli, which can be correlated with the absence or reduced affinity of several Fis-binding sites in the S. typhimurium fis promoter region. Phenotypes of fis mutants include loss of Hin-mediated DNA inversion, cell filamentation, reduced growth rates in rich medium, and increased lag times when the mutants are subcultured after prolonged growth in stationary phase. On the other hand, cells constitutively expressing Fis exhibited normal logarithmic growth but showed a sharp reduction in survival during stationary phase. During the course of these studies, the sigma 28-dependent promoter within the hin-invertible segment that is responsible for fljB (H2) flagellin synthesis was precisely located.

Involvement of the galactosyl-1-phosphate transferase encoded by the Salmonella enterica rfbP gene in O-antigen subunit processing

rfbT of Salmonella enterica LT2 was previously thought, together with rfaL, to be involved in the ligation of polymerized O antigen to core-lipid A, and three mutants were known. We report the mapping of the mutations to rfbP, the galactosyl-1-phosphate transferase gene, which is now shown to encode a bifunctional protein. The mutations which have the former rfbT phenotype are referred to as rfbP(T). We also show that rfbP(T) mutants are not blocked in the ligation step as previously believed but in an earlier step, possibly in flipping the O-antigen subunit on undecaprenyl pyrophosphate from the cytoplasmic to periplasmic face of the cytoplasmic membrane.

Mutations affecting a regulated, membrane-associated esterase in Salmonella typhimurium LT2

Mutations at the apeA locus in Salmonella typhimurium lead to loss of a soluble enzyme ("protease I") that hydrolyzes the chromogenic endoprotease substrate N-acetyl phenylalanine beta-naphthyl ester. We have isolated pseudorevertants of S. typhimurium apeA mutations that have regained the ability to hydrolyze this compound. These pseudorevertants contain mutations (apeR) that lead to overproduction of a membrane-bound esterase different from protease I. The apeR locus is phage P1 cotransducible with ilvC (83 map units) and is unlinked to apeA. Mutations at still another locus, apeE, lead to loss of the membrane-associated esterase. The apeE locus is P1 cotransducible with purE (12 map units). In an apeE-lacZ operon fusion strain, an apeR mutation increases the level of beta-galactosidase approximately 60-fold. We propose that apeR encodes a repressor of apeE. The evidence available suggests that the ApeE protein is not a protease.

Approaches to half-tetrad analysis in bacteria: recombination between repeated, inverse-order chromosomal sequences

In standard bacterial recombination assays, a linear fragment of DNA is transferred to a recipient cell and, at most, a single selected recombinant type is recovered from each merozygote. This contrasts with fungal systems, for which tetrads allow recovery of all meiotic products, including both ultimate recombinant products of an apparent single act of recombination. We have developed a bacterial recombination system in which two recombining sequences are placed in inverse order at widely separated sites in the circular chromosome of Salmonella typhimurium. Recombination can reassort markers between these repeated sequences (double recombination and apparent gene conversion), or can exchange flanking sequences, leading to inversion of the chromosome segment between the recombining sequences. Since two recombinant products remain in the chromosome of a recombinant with an inversion, one can, in principle, approach the capability of tetrad analysis. Using this system, the following observations have been made. (a) When long sequences (40 kb) recombine, conversion frequently accompanies exchange of flanking sequences. (b) When short sequences (5 kb) recombine, conversion rarely accompanies exchange of flanks. (c) Both recA and recB mutations eliminate inversion formation. (d) The frequency of exchanges between short repeats is more sensitive to the distance separating the recombining sequences in the chromosome. The results are presented with the assumption that inversions occur by simple interaction of two sequences in the same circular chromosome. In an appendix we discuss mechanistically more complex possibilities, some of which could also apply to standard fungal systems.

Bacteriophage P22 transduction of integrated plasmids: single-step cloning of Salmonella typhimurium gene fusions

Transcriptional fusions to Salmonella typhimurium chromosomal genes were constructed by integration of a suicide fusion vector into the chromosome by homologous recombination with random cloned chromosomal fragments. We describe here a transductional method using the generalized transducing phage of S. typhimurium, P22, to clone these fusions directly from the bacterial chromosome, in a single step, without the use of restriction enzymes. In this transduction, the phage packages the chromosomal fragment containing the integrated plasmid. Once introduced into the recipient, the plasmid circularizes by homologous recombination between the duplicated region determined by the cloned fragment. Although RecA mediates the majority of these events, the plasmid can circularize in a recA recipient. However, in this case, the event occurs at a much lower frequency and only when the transduction is done at a high multiplicity of infection. In addition to integrated fusion constructs, we also show that autonomously replicating low-copy-number plasmids can be transduced. In this case, transduction is dependent on homologous recombination between the plasmid and the donor chromosome via cloned sequences, in which the transducing particle effectively traps the integrated plasmid.

Transcription from two promoters and autoregulation contribute to the control of expression of the Salmonella typhimurium flagellar regulatory gene flgM

The flgM gene product has been shown to be a negative regulator of flagellin transcription in Salmonella typhimurium (K. L. Gillen and K. T. Hughes, J. Bacteriol. 173:2301-2310, 6453-6459, 1991; K. Ohnishi, K. Kutsukake, H. Suzuki, and T. Iino, Mol. Microbiol. 6:3149-3157, 1992). Mud-lac fusions to the flgM gene were isolated and used to characterize the regulation of flgM gene expression. Transcription of the flgM gene was decreased more than 30-fold in strains with the flagellar master regulatory genes, flhC and flhD, deleted. A class 2 flagellar defect caused a slight increase of flgM gene transcription unless a wild-type copy of the flgM gene was present, in which case transcription was decreased threefold. A deletion in the gene for the alternative sigma factor sigma 28 (FliA) caused a fourfold decrease in flgM expression. Insertional inactivation of a gene upstream of the flgM gene (flgA) in a fliA mutant strain caused transcription of the flgM gene to be decreased to a basal level. Northern (RNA) blot analysis confirmed the presence of two transcripts through the flgM gene, one which initiates upstream of the flgM gene and a second which initiates upstream of the flgA gene.

Use of Salmonella phage P22 for transduction in Escherichia coli

A cosmid (pPR1347) carrying both the rfb gene cluster and the rfc gene of a Salmonella group B serovar has been constructed; Escherichia coli K-12 strains carrying this cosmid produce long-chain O antigen, are sensitive to phage P22, and can be transduced by P22. Some of the benefits of P22 transduction are now available for studying E. coli and potentially other genera.

Recombination-deficient mutants of Salmonella typhimurium are avirulent and sensitive to the oxidative burst of macrophages

Mutations in the genes recA and recBC were constructed in the virulent Salmonella typhimurium strain 14028s. Both the recA and recBC mutants were attenuated in mice. The mutants were also sensitive to killing by macrophages in vitro. The recombination mutants were no longer macrophage sensitive in a variant line of J774 macrophage-like cells that fail to generate superoxide. This suggests that repair of DNA damage by Salmonella is necessary for full virulence in vivo and that the oxidative burst of phagocytes is one source of such DNA damage.

Identification, expression, and DNA sequence of the GDP-mannose biosynthesis genes encoded by the O7 rfb gene cluster of strain VW187 (Escherichia coli O7:K1)

The O7-specific lipopolysaccharide (LPS) in strains of Escherichia coli consists of a repeating unit made of galactose, mannose, rhamnose, 4-acetamido-2,6-dideoxyglucose, and N-acetylglucosamine. We have recently cloned and characterized genetically the O7-specific LPS biosynthesis region (rfbEcO7) of the E. coli O7:K1 strain VW187 (C. L. Marolda, J. Welsh, L. Dafoe, and M. A. Valvano, J. Bacteriol. 172:3590-3599, 1990). In this study, we localized the gnd gene encoding gluconate-6-phosphate dehydrogenase at one end of the rfbEcO7 gene cluster and sequenced that end of the cluster. Three open reading frames (ORF) encoding polypeptides of 275, 464, and 453 amino acids were identified upstream of gndEcO7, all transcribed toward the gnd gene. ORF275 had 45% similarity at the protein level with ORF16.5, which occupies a similar position in the Salmonella enterica LT2 rfb region, and presumably encodes a nucleotide sugar transferase. The polypeptides encoded by ORFs 464 and 453 were expressed under the control of the ptac promoter and visualized in Coomassie blue-stained sodium dodecyl sulfate-polyacrylamide gels and by maxicell analysis. ORF464 expressed GDP-mannose pyrophosphorylase and ORF453 encoded a phosphomannomutase, the enzymes for the biosynthesis pathway of GDP-mannose, one of the nucleotide sugar precursors for the formation of the O7 repeating unit. They were designated rfbMEcO7 and rfbKEcO7, respectively. The RfbMEcO7 polypeptide was homologous to the corresponding protein in S. enterica LT2, XanB of Xanthomonas campestris, and AlgA of Pseudomonas aeruginosa, all GDP-mannose pyrophosphorylases. RfbKEcO7 was very similar to CpsG of S. enterica LT2, an enzyme presumably involved in the biosynthesis of the capsular polysaccharide colanic acid, but quite different from the corresponding RfbK protein of S. enterica LT2.

A single amino acid substitution in elongation factor Tu disrupts interaction between the ternary complex and the ribosome

Elongation factor Tu (EF-Tu).GTP has the primary function of promoting the efficient and correct interaction of aminoacyl-tRNA with the ribosome. Very little is known about the elements in EF-Tu involved in this interaction. We describe a mutant form of EF-Tu, isolated in Salmonella typhimurium, that causes a severe defect in the interaction of the ternary complex with the ribosome. The mutation causes the substitution of Val for Gly-280 in domain II of EF-Tu. The in vivo growth and translation phenotypes of strains harboring this mutation are indistinguishable from those of strains in which the same tuf gene is insertionally inactivated. Viable cells are not obtained when the other tuf gene is inactivated, showing that the mutant EF-Tu alone cannot support cell growth. We have confirmed, by partial protein sequencing, that the mutant EF-Tu is present in the cells. In vitro analysis of the natural mixture of wild-type and mutant EF-Tu allows us to identify the major defect of this mutant. Our data shows that the EF-Tu is homogeneous and competent with respect to guanine nucleotide binding and exchange, stimulation of nucleotide exchange by EF-Ts, and ternary complex formation with aminoacyl-tRNA. However various measures of translational efficiency show a significant reduction, which is associated with a defective interaction between the ribosome and the mutant EF-Tu.GTP.aminoacyl-tRNA complex. In addition, the antibiotic kirromycin, which blocks translation by binding EF-Tu on the ribosome, fails to do so with this mutant EF-Tu, although it does form a complex with EF-Tu. Our results suggest that this region of domain II in EF-Tu has an important function and influences the binding of the ternary complex to the codon-programmed ribosome during protein synthesis. Models involving either a direct or an indirect effect of the mutation are discussed.

The Salmonella typhimurium nadC gene: sequence determination by use of Mud-P22 and purification of quinolinate phosphoribosyltransferase

The Salmonella typhimurium nadC gene and its product, quinolinic acid phosphoribosyltransferase (QAPRTase), were characterized at the molecular and biochemical levels. Fusions of Mud-lac elements isolated in the nadC gene were converted to Mud-P22 insertions. Starting with six original Mud-lac fusions, the entire sequence of the nadC gene was readily obtained. The sequence shows a long open reading frame with two potential initiator methionines, one of which is preceded by the Shine-Dalgarno sequence GGAG-7-nucleotide-ATG. The protein predicted from this second open reading frame is 297 residues in length. The nadC gene was subcloned into a T7-based expression system, allowing for facile purification of the QAPRTase (EC 2.4.2.19) protein to homogeneity. Upon gel filtration, the protein gave an M(r) of 72,000, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis gave a subunit M(r) of 35,000. Automated Edman degradation of several tryptic peptides confirmed the amino acid sequence predicted from the DNA sequence. Chromatography of the apparently homogeneous enzyme on reverse-phase high-performance liquid chromatography resolved two protein species. One of these species failed to give an amino-terminal sequence, while the other yielded the amino-terminal sequence predicted by the second open reading frame and lacked the initiator methionine. The mass of the mature protein, predicted from its DNA sequence, was 32,428 Da. Electrospray mass spectrometry gave masses of 32,501 and 32,581 Da for the two peptides. Steady-state kinetics on the purified QAPRTase indicated Km values of 32 microM for 5-phosphoribosyl-1-pyrophosphate and 20 microM for quinolinate. Vmax was 0.9 U/mg, similar to values reported for this enzyme by other sources.

Invasion by Salmonella typhimurium is affected by the direction of flagellar rotation

When grown aerobically, Salmonella typhimurium exhibits a low level of entry into tissue culture cells. We have isolated an S. typhimurium Tn10 mutant which, when grown under aerobic conditions, efficiently invades HEp-2 cells. Sequencing of S. typhimurium DNA adjacent to the site of the Tn10 element showed that the insertion disrupted transcription of the aspartate receptor gene, tar. Polar effects of the transposon on downstream genes also eliminated chemotaxis. Isogenic nonchemotactic (Che-), as well as nonmotile (Mot-) and nonflagellated (Fla-), S. typhimurium strains were examined for their ability to invade HEp-2 cells. "Smooth" swimming Che- mutants (cheA, cheW, cheR, and cheY) were found to possess increased invasiveness for cultured mammalian cells. In contrast, a "tumbly" cheB mutant and Mot- (flagellated) strain were found to have decreased levels of tissue culture invasiveness. A Fla- strain was found to be as invasive as the wild-type strain if centrifugation was used to facilitate contact with the monolayer surface. In addition, the observed hyperinvasiveness of the smooth swimming tar::Tn10 mutant was suppressed when the strain was paralyzed by the introduction of a mot or fla mutation. A murine infection model was used to demonstrate that the mutant invasive phenotypes were also observed in vivo. These data are most consistent with the idea that the rotation and physical orientation of flagella around the bacteria affect the ability of salmonellae to enter host cells.

The Salmonella typhimurium locus mviA regulates virulence in Itys but not Ityr mice: functional mviA results in avirulence; mutant (nonfunctional) mviA results in virulence

The virulent Salmonella typhimurium strain WB600 carries the mviA allele of the gene mouse virulence A. As shown here, the virulent phenotype of WB600 is the result of a nonfunctional mviA gene. As compared to the functional allele mviA+, mviA increases virulence in Itys mice, but not in Ityr mice. A specific BglII site, mviA4185, between osmZ and galU, located at approximately 35 min on the salmonella chromosome, was within mviA. Insertion of an antibiotic cassette in the mviA4185 site of mviA+ or the homologous mviA4093 site of mviA DNA resulted in virulence when either cassette was recombined into the chromosome. When mviA and mviA+ were both expressed in the same strain with one carried in the chromosome and the other on a plasmid, avirulence was dominant. Replacement of the mviA allele of strain WB600 using P22 transductions of linked antibiotic cassettes cloned into the chromosome of virulent S. typhimurium strains (SR-11, TML, SL1344, C5, ATCC14028, W118-2, and WB600) showed that all but WB600 contained the avirulent mviA+ allele. Southern hybridizations provided no evidence for a second mviA allele anywhere in the genome of the six non-WB600 strains.

Molecular characterization of flgM, a gene encoding a negative regulator of flagellin synthesis in Salmonella typhimurium

The expression of flagellin in Salmonella typhimurium is coupled to the assembly of complete flagella. Mutations which disrupt this coupling define a gene, flgM, which represses the expression of the flagellin genes in strains with mutations in the basal body, switch, or hook flagellar gene (K. L. Gillen and K. T. Hughes, J. Bacteriol. 173:2301-2310, 1991). Complementation studies demonstrated that the flgM gene is contained within a 600-bp cloned DNA fragment. Sequence analysis revealed that this fragment carries a small open reading frame corresponding to a 97-amino-acid protein. The FlgM protein observed in a T7-mediated expression system showed an apparent molecular mass of 9.5 kDa, similar to the predicted size of 10.6 kDa. Upstream of the flgM coding region is a putative promoter sequence which shows strong homology to that thought to be recognized by the flagellin-specific sigma factor (FliA). Consistent with the use of this promoter in vivo, promoter mapping by primer extension demonstrated a transcriptional start site 11 bases downstream from the center of the putative -10 promoter element, which was dependent on functional FliA for full expression.

Mutant ribosomes can generate dominant kirromycin resistance

Mutations in the two genes for EF-Tu in Salmonella typhimurium and Escherichia coli, tufA and tufB, can confer resistance to the antibiotic kirromycin. Kirromycin resistance is a recessive phenotype expressed when both tuf genes are mutant. We describe a new kirromycin-resistant phenotype dominant to the effect of wild-type EF-Tu. Strains carrying a single kirromycin-resistant tuf mutation and an error-restrictive, streptomycin-resistant rpsL mutation are resistant to high levels of kirromycin, even when the other tuf gene is wild type. This phenotype is dependent on error-restrictive mutations and is not expressed with nonrestrictive streptomycin-resistant mutations. Kirromycin resistance is also expressed at a low level in the absence of any mutant EF-Tu. These novel phenotypes exist as a result of differences in the interactions of mutant and wild-type EF-Tu with the mutant ribosomes. The restrictive ribosomes have a relatively poor interaction with wild-type EF-Tu and are thus more easily saturated with mutant kirromycin-resistant EF-Tu. In addition, the mutant ribosomes are inherently kirromycin resistant and support a significantly faster EF-Tu cycle time in the presence of the antibiotic than do wild-type ribosomes. A second phenotype associated with combinations of rpsL and error-prone tuf mutations is a reduction in the level of resistance to streptomycin.

Cloning and nucleotide sequence of the anaerobically regulated pepT gene of Salmonella typhimurium

The anaerobically regulated pepT gene of Salmonella typhimurium has been cloned in pBR328. Strains carrying the pepT plasmid, pJG17, overproduce peptidase T by approximately 70-fold. The nucleotide sequence of a 2.5-kb region including pepT has been determined. The sequence codes for a protein of 44,855 Da, consistent with a molecular weight of approximately 46,000 for peptidase T (as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and gel filtration). The N-terminal amino acid sequence of peptidase T purified from a pJG17-containing strain matches that predicted by the nucleotide sequence. A plasmid carrying an anaerobically regulated pepT::lacZ transcriptional fusion contains only 165 bp 5' to the start of translation. This region contains a sequence highly homologous to that identified in Escherichia coli as the site of action of the FNR protein, a positive regulator of anaerobic gene expression. A region of the deduced amino acid sequence of peptidase T is similar to segments of Pseudomonas carboxypeptidase G2, the E. coli peptidase encoded by the iap gene, and E. coli peptidase D.

The cps gene cluster of Salmonella strain LT2 includes a second mannose pathway: sequence of two genes and relationship to genes in the rfb gene cluster

We report the presence in Salmonella enterica strain LT2 (serovar thyphimurium) of duplicate genes for two steps in the synthesis of GDP-mannose. The previously known genes, rfbK (phosphomannomutase) and rfbM (mannose-1-phosphate guanyltransferase), are part of the gene cluster for the O antigen. The two new genes, cpsB and cpsG, respectively, are thought to be part of the gene cluster for the M antigen capsular polysaccharide, present in many Enterobacteriaceae. The two genes have been sequenced and have a GC content of 0.61, suggesting an origin outside of Salmonella. Comparison of the inferred protein sequences for cpsB and rfbM shows 57% identity of amino acids whereas for cpsG and rfbK there is only 19% identity. It is suggested that the greater divergence between cpsG and rfbK may be due to a period of accelerated evolution, perhaps precipitated by transfer of the genes from another species.

Negative regulatory loci coupling flagellin synthesis to flagellar assembly in Salmonella typhimurium

The complex regulation of flagellin gene expression in Salmonella typhimurium was characterized in vivo by using lac transcriptional fusions to the two flagellin structural genes (fliC [H1] and fljB [H2]). Phase variation was measured as the rate of switching of flagellin gene expression. Switching frequencies varied from 1/500 per cell per generation to 1/10,000 per cell per generation depending on the particular insertion and the direction of switching. There is a 4- to 20-fold bias in favor of switching from the fljB(On) to the fljB(Off) orientation. Random Tn10dTc insertions were isolated which failed to express flagellin. While most of these insertions mapped to loci known to be required for flagellin expression, several new loci were identified. The presence of functional copies of all of the genes responsible for complete flagellar assembly, except the hook-associated proteins (flgK, flgL, and fliD gene products), were required for expression of the fliC or fljB flagellin genes. Two novel loci involved in negative regulation of fliC and fljB in fla mutant backgrounds were identified. One of these loci, designated the flgR locus, mapped to the flg operon at 23 min on the Salmonella linkage map. An flgR insertion mutation resulted in relief of repression of the fliC and fljB genes in all fla mutant backgrounds except for mutants in the positive regulatory loci (flhC, flhD, and fliA genes).

A genetic characterization of the nadC gene of Salmonella typhimurium

The nadC gene of Salmonella encodes the pyridine biosynthetic enzyme PRPP-quinolinate phosphoribosyltransferase. Using a combination of genetic techniques, a deletion map for the Salmonella nadC gene has been generated which includes over 100 point mutants and 18 deletion intervals. The nadC alleles obtained by hydroxylamine mutagenesis include those suppressed by either amber, ochre, or UGA nonsense suppressors as well as alleles suppressed by the missense suppressor, sumA. Deletions were obtained by three separate protocols including spontaneous selection for loss of the nearby aroP gene, recombination between aroP::MudA and nadC::MudA insertion alleles, and selection for spontaneous loss of tetracycline resistance in a nearby guaC::Tn10dTc insertion mutant allele. The nadC mutants comprise one complementation group and the nadC+ allele is dominant to simple, nadC auxotrophic mutant alleles. Intragenic complementation of two nadC alleles, nadC493 and nadC494, mapping to deletion intervals 17 and 18, respectively, suggests that nadC encodes a multimeric enzyme. Both nadC and the nearby aroP locus are transcribed counterclockwise on the standard genetic map of Salmonella, in opposite orientation to the direction of chromosome replication.

Plasmid-mediated sucrose metabolism in Escherichia coli: characterization of scrY, the structural gene for a phosphoenolpyruvate-dependent sucrose phosphotransferase system outer membrane porin

The scrY gene, part of the pUR400-borne sucrose regulon, appeared to be transcribed from its own promoter, with the transcriptional start site located 58 bp upstream from the initiation codon. An open reading frame encoding a polypeptide of 505 amino acid residues (Mr 55,408) was identified. The first 22 amino acid residues formed a leader sequence typical of those found in other procaryotic outer membrane and periplasmic proteins. A frameshift mutation in the scrY gene resulted in a dramatic decrease in sucrose transport with no effect on in vitro phosphorylation activity associated with enzyme IISer. The rate of diffusion of sucrose was 96 times greater than the rate of diffusion of lactose or maltose in liposomes containing the ScrY protein. This increase in sucrose permeability provided strong evidence that the ScrY protein functions as a sucrose porin. There was 23% amino acid sequence identity between the ScrY protein and LamB, a maltose porin from Escherichia coli.

Overproduction of release factor reduces spontaneous frameshifting and frameshift suppression by mutant elongation factor Tu

Mutant forms of elongation factor Tu encoded by tufA8 and tufB103 in Salmonella typhimurium cause suppression of some but not all frameshift mutations. All of the suppressed mutations in S. typhimurium have frameshift windows ending in the termination codon UGA. Because both tufA8 and tufB103 are moderately efficient UGA suppressors, we asked whether the efficiency of frameshifting is influenced by the level of misreading at UGA. We introduced plasmids synthesizing either one of the release factors into strains in which the tuf mutations suppress a test frameshift mutation. We found that overproduction of release factor 2 (which catalyzes release at UGA and UAA) reduced frameshifting promoted by the tuf mutations at all sites tested. However, at one of these sites, trpE91, overproduction of release factor 1 also reduced suppression. The spontaneous level of frameshift "leakiness" at three sites in trpE, each terminating in UGA, was reduced in strains carrying the release factor 2 plasmid. We conclude that both spontaneous and suppressor-enhanced reading-frame shifts are influenced by the activity of peptide chain release factors. However, the data suggest that the effect of release factor on frameshifting does not necessarily depend on the presence of the normal triplet termination signal.

Induction of genetic duplications and frameshift mutations in Salmonella typhimurium by acridines and acridine mustards: dependence on covalent binding of the mutagen to DNA

The aroC321 allele permits positive selection for the detection of a large genetic duplication that arises in the Salmonella typhimurium chromosome by homologous recombination. Strains that contain both aroC321 and the hisC3076 allele were constructed so that the induction of genetic duplications and frameshift mutations in a run of GC base pairs could be studied simultaneously by selecting for tryptophan and histidine prototrophy, respectively. Using these strains, we examined the ability of 9-aminoacridine, quinacrine, four acridine mustards (ICR-170, ICR-191, ICR-372, and quinacrine mustard) and the nitroacridine Entozon to induce genetic duplications and frameshift mutations. Although all these compounds induce reversion of hisC3076, only the four mustards and Entozon are effective as inducers of genetic duplications under identical treatment conditions. The induction of genetic duplications by acridine mustards, like the toxic and mutagenic effects of these compounds, is enhanced by a deficiency for excision repair caused by a deletion through the uvr B gene. The ineffectiveness of 9-aminoacridine and quinacrine in the test for genetic duplications indicates that simple intercalation is sufficient for the mutagenic effect measured with the hisC3076 allele but that the induction of duplications by the acridine mustards and Entozon requires covalent binding of the chemical to DNA.

Transcription of pfl is regulated by anaerobiosis, catabolite repression, pyruvate, and oxrA: pfl::Mu dA operon fusions of Salmonella typhimurium

Pyruvate formate-lyase (EC 2.3.1.54), a key enzyme in the anaerobic metabolism of Salmonella typhimurium, catalyzes the conversion of pyruvate to acetyl coenzyme A and formate. pfl::Mu dA operon fusions were isolated for the study of transcriptional regulation. pfl was transcribed both aerobically and anaerobically, but the activity increased about sixfold under anaerobic conditions. The addition of pyruvate, formate, and acetate in nutrient broth did not have any effect on the anaerobic expression of pfl. However, the addition of pyruvate to minimal glucose medium increased the anaerobic expression of pfl. The expression of pfl varied in different growth media. Anaerobic expression of pfl was lower when the culture was grown in minimal glucose medium than when it was grown in nutrient broth. When Casamino Acids (Difco Laboratories, Detroit, Mich.) were added to minimal glucose medium, the expression of pfl increased proportionally with the amount of Casamino Acids added. The transcription of pfl was positively controlled by the oxrA gene product and was affected by both the cya and crp mutations. However, mutations in genes affecting the cyclic AMP-cyclic AMP receptor protein complex or oxrA could not completely abolish the anaerobic derepression of pfl. In merodiploid strains, pfl::Mu dA/F' pfl+, the beta-galactosidase activities were decreased. The mutations gyrA, oxrC, and oxrE, which affected anaerobic metabolism, did not affect anaerobic expression of pfl.

Characterization of two mutant metJ proteins with reduced, temperature-dependent capacity to regulate Escherichia coli K-12 met regulon elements

At 28 degrees C, but not at 34 or 42 degrees C, strains with the metJ193 allele repressed chromosomal met genes but not a plasmid-borne met promoter. Increasing the metJ193 gene dosage to two copies resulted in overrepression of chromosomal and plasmid-borne met promoters at 28 degrees C. Suppressing the metJ185 amber mutation with supF (tRNATyr) produced the MetJ185F protein. Strains producing MetJ185F repressed chromosomal met promoters but not a plasmid-borne met promoter at 42 degrees C. These are the first known defective MetJ proteins with documented temperature-dependent function.

Altered transcriptional patterns affecting several metabolic pathways in strains of Salmonella typhimurium which overexpress the fructose regulon

Expression of beta-galactosidase in transcriptional fusions with the pps gene (encoding phosphoenolpyruvate [PEP] synthase), the aceBAK operon (encoding malate synthase, isocitrate lyase, and isocitrate dehydrogenase kinase, respectively), and the phs operon (encoding either thiosulfate reductase or a regulatory protein controlling its expression) was studied in Salmonella typhimurium. beta-Galactosidase synthesis in these strains was repressible either by growth in the presence of glucose or by the presence of a fruR mutation, which resulted in the constitutive expression of the fructose (fru) regulon. Five enzymes of gluconeogenesis (PEP synthase, PEP carboxykinase, isocitrate lyase, malate synthase, and fructose-1,6-diphosphatase) were shown to be repressed either by growth in the presence of glucose or the fruR mutation, while the glycolytic enzymes, enzyme I and enzymes II of the phosphotransferase system as well as phosphofructokinase, were induced either by growth in the presence of glucose or the fruR mutation. Overexpression of the cloned fru regulon genes (not including fruR) resulted in parallel repression of representative gluconeogenic, Krebs cycle, and glyoxylate shunt enzymes. Studies with temperature-sensitive mutants of S. typhimurium which synthesized heat-labile IIIFru proteins provided evidence that this protein plays a role in the regulation of gluconeogenic substrate utilization. Other mutant analyses revealed a complex relationship between fru gene expression and the expression of genes encoding gluconeogenic enzymes. Taken together, the results suggest that a number of genes encoding catabolic, biosynthetic, and amphibolic enzymes in enteric bacteria are transcriptionally regulated by a complex catabolite repression/activation mechanism which may involve enzyme IIIFru of the phosphotransferase system as one component of the regulatory system.

Absence of insertions among spontaneous mutants of Salmonella typhimurium

While insertion sequences (IS) in Escherichia coli transpose frequently to generate spontaneous insertion mutants, such mutations are rare in Salmonella typhimurium: the only documented insertion mutation is a hisD mutation caused by the Salmonella-specific IS element IS200. To obtain more examples of IS200 insertion mutations and to seek additional types of IS elements in Salmonella, we selected and characterized 422 independent, spontaneous His- mutants and some 2100 additional mutants that are not necessarily independent. None of the mutants showed the absolute polar effect characteristic of insertion mutations or the reversion properties characteristic of insertions (low spontaneous reversion frequency and no reversion induction by chemical mutagens). A few mutants, showing a high spontaneous reversion frequency, were screened physically. No insertion mutations were found. Thus insertion mutations appear to be rare in S. typhimurium, in strong contrast to E. coli and despite the possession in Salmonella of at least one type of insertion element (IS200). These results suggest that in Salmonella transposition of the endogenous elements has been controlled. The transposition ability of the elements may have been reduced or favored target sites removed from the host genome.

DNA sequence of the metC gene and its flanking regions from Salmonella typhimurium LT2 and homology with the corresponding sequence of Escherichia coli

The DNA sequence of the Salmonella typhimurium metC gene and its flanking regions was determined. The metC gene contains an open reading frame of 1185 nucleotides encoding a polypeptide of 395 amino acids with a predicted molecular weight of 42,874 daltons. S1 nuclease mapping experiments located the transcription start site of the metC gene. The nucleotide sequence and the deduced amino acid sequence for the metC genes of S. typhimurium and Escherichia coli were compared. Although there are 279 nucleotide replacements, most do not change the amino acid sequence. Nucleotide sequence analysis of the flanking regions of the S. typhimurium metC gene shows that there is an open reading frame upstream and an open reading frame downstream of the gene. The existence of the divergently transcribed upstream open reading frame (designated ORF1) was confirmed by the construction of an ORF1-lacZ fusion. The transcription start site of ORF1 was determined by S1 nuclease mapping.

A collection of strains containing genetically linked alternating antibiotic resistance elements for genetic mapping of Escherichia coli

We present a collection of 182 isogenic strains containing genetically linked antibiotic resistance elements located at approximately 1-min intervals around the Escherichia coli chromosome. At most positions both Tn10 (Tetr) and TN10kan (Kanr) elements are available, so that the collection contains a linked set of alternating antibiotic resistance markers. The map position of each insertion has been aligned to the E. coli genetic map as well as to the Kohara ordered clone bank. These strains are designed to be used in a rapid two-step mapping system in E. coli. In the first step, the mutation is localized to a 5- to 15-min region of the chromosome by Hfr mapping with a set of Hfr strains containing either Tn10 or Tn10kan elements located 20 min from their respective origins of transfer. In the second step, the mutation is localized to a 1-min region by P1 transduction, with a collection of isogenic insertion strains as donors. We discuss the uses of this collection of strains to map and eventually to clone a variety of mutations in E. coli.