Mutations in multicopy Tn10 tet plasmids that confer resistance to inhibitory effects of inducers of tet gene expression

Programmable bacteria detect and record an environmental signal in the mammalian gut

The mammalian gut is a dynamic community of symbiotic microbes that interact with the host to impact health, disease, and metabolism. We constructed engineered bacteria that survive in the mammalian gut and sense, remember, and report on their experiences. Based on previous genetic memory systems, we constructed a two-part system with a "trigger element" in which the lambda Cro gene is transcribed from a tetracycline-inducible promoter, and a "memory element" derived from the cI/Cro region of phage lambda. The memory element has an extremely stable cI state and a Cro state that is stable for many cell divisions. When Escherichia coli bearing the memory system are administered to mice treated with anhydrotetracycline, the recovered bacteria all have switched to the Cro state, whereas those administered to untreated mice remain in the cI state. The trigger and memory elements were transferred from E. coli K12 to a newly isolated murine E. coli strain; the stability and switching properties of the memory element were essentially identical in vitro and during passage through mice, but the engineered murine E. coli was more stably established in the mouse gut. This work lays a foundation for the use of synthetic genetic circuits as monitoring systems in complex, ill-defined environments, and may lead to the development of living diagnostics and therapeutics.

Excision of an unstable pathogenicity island in Salmonella enterica serovar Enteritidis is induced during infection of phagocytic cells

The availability of the complete genome sequence of several Salmonella enterica serovars has revealed the presence of unstable genetic elements in these bacteria, such as pathogenicity islands and prophages. This is the case of Salmonella enterica serovar Enteritidis (S. Enteritidis), a bacterium that causes gastroenteritis in humans and systemic infection in mice. The whole genome sequence analysis for S. Enteritidis unveiled the presence of several genetic regions that are absent in other Salmonella serovars. These regions have been denominated “regions of difference” (ROD). In this study we show that ROD21, one of such regions, behaves as an unstable pathogenicity island. We observed that ROD21 undergoes spontaneous excision by two independent recombination events, either under laboratory growth conditions or during infection of murine cells. Importantly, we also found that one type of excision occurred at higher rates when S. Enteritidis was residing inside murine phagocytic cells. These data suggest that ROD21 is an unstable pathogenicity island, whose frequency of excision depends on the environmental conditions found inside phagocytic cells.

C-terminal binding protein and poly(ADP)ribose polymerase 1 contribute to repression of the p21(waf1/cip1) promoter

Transcriptional repression by the C-terminal binding protein (CtBP) is proposed to require nicotinamide adenine dinucleotide dehydrogenase (NAD(H). Previous studies have implicated CtBP in transcriptional repression of the p21(waf1/cip1) gene. Similarly, the NAD-dependent poly(adenosine diphosphate)ribose polymerase 1 (PARP1) may affect p21 expression via its NAD-dependent enzymatic activity; we therefore asked if PARP1 and CtBP were functionally linked in regulating p21 transcription. We found that restraint of basal p21 transcription requires both CtBP and PARP1. PARP inhibition attenuated activation of p21 transcription by both p53-independent and p53-dependent processes, in a CtBP-dependent manner. CtBP1+2 or PARP1+2 knockdown partially activated p21 gene expression, suggesting relief of a corepressor function dependent on both proteins. We localized CtBP-responsive repression elements to the proximal promoter region, and found ZBRK1 overexpression could also overcome DNA damage-dependent, but not p53-dependent activation through this region. By chromatin immunoprecipitation we find dismissal of CtBP from the proximal promoter following DNA-damage, and that PARP1 associates with a CtBP corepressor complex in nuclear extracts. We propose a model in which both CtBP and PARP functionally interact in a corepressor complex as components of a molecular switch necessary for p21 repression, and following DNA damage signals activation of p21 transcription by corepressor dismissal and co-activator recruitment.

Inactivation of phospholipase D diminishes Acinetobacter baumannii pathogenesis

Acinetobacter baumannii is an emerging bacterial pathogen of considerable health care concern. Nonetheless, relatively little is known about the organism's virulence factors or their regulatory networks. Septicemia and ventilator-associated pneumonia are two of the more severe forms of A. baumannii disease. To identify virulence factors that may contribute to these disease processes, genetically diverse A. baumannii clinical isolates were evaluated for the ability to proliferate in human serum. A transposon mutant library was created in a strain background that propagated well in serum and screened for members with decreased serum growth. The results revealed that disruption of A. baumannii phospholipase D (PLD) caused a reduction in the organism's ability to thrive in serum, a deficiency in epithelial cell invasion, and diminished pathogenesis in a murine model of pneumonia. Collectively, these results suggest that PLD is an A. baumannii virulence factor.

Metabolite profiling uncovers plasmid-induced cobalt limitation under methylotrophic growth conditions

BACKGROUND

The introduction and maintenance of plasmids in cells is often associated with a reduction of growth rate. The reason for this growth reduction is unclear in many cases.

METHODOLOGY/PRINCIPAL FINDINGS

We observed a surprisingly large reduction in growth rate of about 50% of Methylobacterium extorquens AM1 during methylotrophic growth in the presence of a plasmid, pCM80 expressing the tetA gene, relative to the wild-type. A less pronounced growth delay during growth under non-methylotrophic growth conditions was observed; this suggested an inhibition of one-carbon metabolism rather than a general growth inhibition or metabolic burden. Metabolome analyses revealed an increase in pool sizes of ethylmalonyl-CoA and methylmalonyl-CoA of more than 6- and 35-fold, respectively, relative to wild type, suggesting a strongly reduced conversion of these central intermediates, which are essential for glyoxylate regeneration in this model methylotroph. Similar results were found for M. extorquens AM1 pCM160 which confers kanamycin resistance. These intermediates of the ethylmalonyl-CoA pathway have in common their conversion by coenzyme B(12)-dependent mutases, which have cobalt as a central ligand. The one-carbon metabolism-related growth delay was restored by providing higher cobalt concentrations, by heterologous expression of isocitrate lyase as an alternative path for glyoxylate regeneration, or by identification and overproduction of proteins involved in cobalt import.

CONCLUSIONS/SIGNIFICANCE

This study demonstrates that the introduction of the plasmids leads to an apparent inhibition of the cobalt-dependent enzymes of the ethylmalonyl-CoA pathway. Possible explanations are presented and point to a limited cobalt concentration in the cell as a consequence of the antibiotic stress.

Rational vector design for efficient non-viral gene delivery: challenges facing the use of plasmid DNA

Although non-viral gene delivery is a very straightforward technology, there are currently no FDA-approved gene medicinal products available. Therefore, improving potency, safety, and efficiency of current plasmid DNA vectors will be a major task for the near future. This article will provide an overview on factors influencing production yield and quality as well as safety issues that emerge from the vector design itself. Special focus will be on generating bacterial pDNA vectors by circumventing the use of antibiotic resistance genes, to generate safer gene medicinal products as well as smaller, more efficient DNA vectors.

Persister cells and tolerance to antimicrobials

Bacterial populations produce persister cells that neither grow nor die in the presence of microbicidal antibiotics. Persisters are largely responsible for high levels of biofilm tolerance to antimicrobials, but virtually nothing was known about their biology. Tolerance of Escherichia coli to ampicillin and ofloxacin was tested at different growth stages to gain insight into the nature of persisters. The number of persisters did not change in lag or early exponential phase, and increased dramatically in mid-exponential phase. Similar dynamics were observed with Pseudomonas aeruginosa (ofloxacin) and Staphylococcus aureus (ciprofloxacin and penicillin). This shows that production of persisters depends on growth stage. Maintaining a culture of E. coli at early exponential phase by reinoculation eliminated persisters. This suggests that persisters are not at a particular stage in the cell cycle, neither are they defective cells nor cells created in response to antibiotics. Our data indicate that persisters are specialized survivor cells.

The Salmonella enterica sv. Typhimurium smvA, yddG and ompD (porin) genes are required for the efficient efflux of methyl viologen

In Gram-negative bacteria, a subset of inner membrane proteins in the major facilitator superfamily (MFS) acts as efflux pumps to decrease the intracellular concentrations of multiple toxic substrates and confers multidrug resistance. The Salmonella enterica sv. Typhimurium smvA gene encodes a product predicted to be an MFS protein most similar to QacA of Staphylococcus aureus. Like mutations in qacA, mutations in smvA confer increased sensitivity to methyl viologen (MV). Mutations in the adjacent ompD (porin) and yddG (drug/metabolite transporter) genes also confer increased sensitivity to MV, and mutations in smvA are epistatic to mutations in ompD or yddG for this phenotype. YddG and OmpD probably comprise a second efflux pump in which the OmpD porin acts as an outer membrane channel (OMC) protein for the efflux of MV and functions independently of the SmvA pump. In support of this idea, the pump dependent on YddG and OmpD has a different substrate specificity from the pump dependent on SmvA. Mutations in tolC, which encodes an OMC protein, confer increased resistance to MV. TolC apparently facilitates the import of MV, and a subset of OMC proteins including the OmpD porin and TolC may facilitate both import and export of distinct subsets of toxic substrates.

Regulation of the "tetCD" genes of transposon Tn10

In addition to the genes involved in tetracycline resistance, the loop region of the composite transposon Tn10 contains two other known genes, tetC and tetD, whose functions are unclear. Using primarily a genetic approach, we examined tetCD gene expression and regulation. The tetC gene product, TetC, is a diffusible repressor of both tetC and tetD transcription. Despite an earlier claim by others, we do not detect induction of either tetC or tetD by tetracycline (Tc) or several of its analogs. Although the 5' ends of the tetC and tetD messages overlap due to transcription from convergent promoters, we find no evidence for anti-sense RNA control. The operator for the TetC repressor has been localized. We also demonstrate that transcription from the tetD promoter probably terminates within IS10-Right and does not apparently interfere with Tn10 or IS10-Right transposition or its regulation.

Genetic analysis of a plasmid-encoded, host genotype-specific enhancement of bacterial fitness

In the absence of antibiotics, carriage of pACYC184 reduces the competitive fitness of an Escherichia coli B genotype that was not previously selected for plasmid carriage, relative to that of an isogenic plasmid-free competitor. However, a host genotype propagated with the plasmid for 500 generations evolved an unexpected competitive advantage from plasmid carriage, relative to its own isogenic plasmid-free segregant. We manipulated the pACYC184 genome in order to identify the plasmid-encoded function that was required for the enhancement of the coevolved host genotype's competitive fitness. Inactivation of the plasmid-encoded tetracycline resistance gene, by deletion of either the promoter region or the entire gene, eliminated the beneficial effect of plasmid carriage for the coevolved host. This beneficial effect for the coevolved host was also manifest with pBR322, which contains a tetracycline resistance gene identical to that of pACYC184 but is otherwise heterologous.

Epistatic effects of promoter and repressor functions of the Tn10 tetracycline-resistance operon of the fitness of Escherichia coli

We have been studying the effects of expression of plasmid-borne, Tn10-encoded, tetracycline resistance on the fitness of Escherichia coli K12. We previously demonstrated large reductions in fitness resulting from induced or constitutive expression of the resistance protein; however, any residual expression by the repressed operon was so slight that possession of an inducible resistance function imposed essentially no burden in the absence of antibiotic. Here, we demonstrate two distinct disadvantages for inducible genotypes relative to isogenic constitutive constructs. During the transition from antibiotic-free to antibiotic-containing media, the inducible genotype experiences a longer lag phase prior to growth. In the sustained presence of antibiotic, full induction of the resistance function in the inducible genotype is prevented by the continued action of its repressor. However, these disadvantages may be reduced by increasing the strength of the promoter for the resistance gene in the inducible genotype. Simultaneous consideration of the mode of gene regulation (i.e. constitutive or inducible) and the strength of the resistance-gene promoter (i.e. maximum level of expression) indicates an adaptive landscape with very strong epistasis and, perhaps, multiple fitness peaks.

Sequence of a class E tetracycline resistance gene from Escherichia coli and comparison of related tetracycline efflux proteins

We determined the nucleotide sequence of the class E tetA gene on plasmid pSL1456 from Escherichia coli SLH1456A. The deduced amino acid sequence of the class E TetA protein shows 50 to 56% identity with the sequences of five related TetA proteins (classes A through D and G). Hydrophobicity profiles identify 12 putative transmembrane segments with similar boundaries in all six TetA sequences. The N-terminal alpha domain of the six sequences is more highly conserved than the C-terminal beta domain; the central hydrophilic loop connecting the alpha and beta domains is the least conserved region. Amino acid residues that have been shown to be important for class B (Tn10) TetA function are conserved in all six TetA sequences. Unlike the class B tetA gene, the class D and E tetA genes do not exhibit a negative gene dosage effect when present on multicopy plasmids derived from pACYC177.

Expression of the tetK gene from Staphylococcus aureus in Escherichia coli: comparison of substrate specificities of TetA(B), TetA(C), and TetK efflux proteins

The tetK gene, which encodes a tetracycline efflux pump from Staphylococcus aureus, was expressed in Escherichia coli by using an inducible, low-level expression system. The tetK gene, as well as the tetA(B) gene from the transposon Tn10 and the tetA(C) gene from plasmid pBR322, was subjected to the regulatory control of the lac repressor, and resistance to tetracycline was measured as a function of the isopropyl-beta-D-thiogalactopyranoside concentration. The maximum resistance of the E. coli strain containing the tetK construct was comparable to the maximum resistance of the strain containing the tetA(C) construct but was less than the resistance of the strain containing the tetA(B) construct. Overexpression of the tetK, tetA(B), or tetA(C) genes was toxic. When expression was regulated so that resistance to tetracycline was comparable, then the TetA(B) and TetA(C) proteins conferred very similar levels of resistance to a variety of tetracycline derivatives. In contrast, the TetK protein was less capable of conferring resistance to the tetracycline derivatives minocycline, 6-deoxy-6-demethyltetracycline, and doxycycline. The implications for the recognition of various tetracycline substituents by the TetK protein are discussed.

Plasmid instabilities of single and three-plasmid systems in Escherichia coli during continuous cultivation

Plasmid instabilities in E. coli JM103 carrying three plasmids (pRK248cI, pMTC48, pEcoR4) and a single plasmid system (pTG206) for the production of fusion EcoRI (SPA::EcoRI) and catechol 2,3-dioxygenase, respectively, were investigated in continuous cultures under selective and non-selective conditions. In a three-plasmid system, pRK248cI was lost gradually together with pMTC48 from the host under non-selective conditions. The selective pressure against pRK248cI stabilized the pMTC48. This indicates that the loss of pMTC48 under non-selective conditions was caused by the loss of cI857 gene (coded by pRK248cI) which resulted in the overproduction of the toxic gene product (coded by pMTC48). In the case of single plasmid (pTG206) system, the plasmid lost from the host under non-selective conditions. This plasmid was stabilized in the host growing under selective conditions. During this period we obtained some ampicillin resistant colonies which gave low levels of enzyme activities compared to the normal plasmid bearing cells. Plasmid analysis from the above cells showed that the plasmid has undergone structural instability. Further, restriction analysis of this plasmid exhibited an additional PvuII site in a 0.9 kbp fragment that was integrated near the tet promoter which controls the expression of the xyl E gene, thereby resulting low levels of enzyme activities. Our results indicate that some of the IS elements which are present in the host chromosome were responsible for such instabilities to turn off the synthesis by inserting into the tet promoter region to lower the protein formation during the bioprocess.

Efflux-mediated antiseptic resistance gene qacA from Staphylococcus aureus: common ancestry with tetracycline- and sugar-transport proteins

Resistance to intercalating dyes (ethidium, acriflavine) and other organic cations, such as quaternary ammonium-type antiseptic compounds, mediated by the Staphylococcus aureus plasmid pSK1 is specified by an energy-dependent export mechanism encoded by the qacA gene. From nucleotide sequence analysis, qacA is predicted to encode a protein of Mr 55017 containing 514 amino acids. The gene is likely to initiate with a CUG codon, and a 36 bp palindrome immediately preceding qacA, along with an upstream reading frame with homology to the TetR repressors, may be components of a regulatory circuit. The putative polypeptide specified by qacA has properties typical of a cytoplasmic membrane protein, and is indicated to be a member of a transport protein family that includes proteins responsible for export-mediated resistance to tetracycline and methylenomycin, and uptake of sugars and quinate. The analysis suggests that N- and C-terminal regions of these proteins are involved in energy coupling (proton translocation) and substrate transport, respectively. The last common ancestor of the qacA and related tet (tetracycline resistance) lineages is inferred to have been repressor controlled, as occurs for modern tet determinants from Gram-negative, but not those from Gram-positive, bacteria.

Overproduction and purification of the Tn10-specified inner membrane tetracycline resistance protein Tet using fusions to beta-galactosidase

Tetracycline resistance in the Enterobacteriaceae is mediated by a number of genetically related, usually plasmid-borne, determinants which specify an efflux system involving an inner membrane protein, Tet. Attempts to overproduce the Tn10 (Class B)-encoded Tet in Escherichia coli by cloning the structural gene tet downstream of the lambda PL promoter under regulation by temperature-sensitive lambda repressor cI857 were unsuccessful; induction at 42 degrees C resulted in filamentous, non-viable cells containing little detectable overproduction of the protein. However, cells containing tet fused to lacZ were resistant to tetracycline at 30 degrees C and synthesized modest amounts of a large fusion protein when induced at 42 degrees C. Fusion of the N-terminal half or the first 38 amino acids of tet to lacZ did lead to increased production of fusion proteins. Fusions could be purified by size or by LacZ immunoaffinity or substrate-affinity chromatography. In the latter method, selected detergents were required to counteract nonspecific binding of Tet to the adsorbant. Amino acid sequencing of the N-terminus of Tet-LacZ fusion proteins indicated that most molecules were blocked at this terminus. The sequence of an unblocked subpopulation was consistent with that expected from the nucleotide sequence. A collagen peptide linker, genetically placed between tet and lacZ, allowed recovery of purified Tet protein after collagenase treatment of the purified fusion protein.

Overproduction of transposon Tn10-encoded tetracycline resistance protein results in cell death and loss of membrane potential

High-level expression of the Tn10 tetracycline resistance protein TetA in Escherichia coli caused partial collapse of the membrane potential, arrest of growth, and killing of the cells. Since alpha-methylglucoside transport was not affected, the overproduced TetA protein may cause not destruction of membrane structure but rather unrestricted translocation of protons and/or ions across the membrane.

Mutation and selection in bacterial populations: alternatives to the hypothesis of directed mutation

Bacterial populations have served as model systems for studying evolutionary processes ever since the classic experiments of Luria and Delbrück, which demonstrated the occurrence of mutations prior to selection for the traits they conferred. However, several authors have recently presented experiments suggesting that bacteria may have mechanisms for directing which mutations occur, such that the rate of adaptive mutations is enhanced. Before the hypothesis of directed mutation is accepted, it is imperative to consider alternative hypotheses that might account for the same observations. To this end, we expand upon existing mathematical theory of the dynamics of mutation and selection in clonal populations for two cases of particular interest. The first case concerns selection against mutants before plating; this selection occurs as the result of differences in growth rate between mutants and nonmutants. We demonstrate that this selection model gives rise to distributions of mutants, obtained by plating from sister cultures, that are very similar to those expected when some mutations are induced by the selective environment. The second case concerns the sequential incorporation of two mutations as the result of selection for an intermediate genotype after plating. We demonstrate that this two-step mutation model also yields distributions that are similar to those expected when some mutations are induced by the selective environment. These two cases therefore provide alternatives to the hypothesis of directed mutation. We suggest experiments that might be used to examine our alternative hypotheses. We also contrast the hypothesis of directed mutation with the notion of inheritance of acquired characteristics.

Antisense RNA: effect of ribosome binding sites, target location, size, and concentration on the translation of specific mRNA molecules

A series of plasmids were constructed to generate RNA complementary to the beta-galactosidase messenger RNA under control of the phage lambda PL promoter. These plasmids generate anti-lacZ mRNA bearing or lacking a synthetic ribosome binding site adjacent to the lambda PL promoter and/or the lacZ ribosome binding site in reverse orientation. Fragments of lacZ DNA from the 5' and/or the 3' region were used in these constructions. When these anti-mRNA molecules were produced in Escherichia coli 294, maximal inhibition of beta-galactosidase synthesis occurred when a functional ribosome binding site was present near the 5' end of the anti-mRNA and the anti-mRNA synthesized was complementary to the 5' region of the mRNA corresponding to the lacZ ribosome binding site and/or the 5'-coding sequence. Anti-mRNAs producing maximal inhibition of beta-galactosidase synthesis exhibited an anti-lacZ mRNA:normal lacZ mRNA ratio of 100:1 or higher. Those showing lower levels of inhibition exhibited much lower anti-lacZ mRNA:normal lacZ mRNA ratios. A functional ribosome binding site at the 5'-end was found to decrease the decay rate of the anti-lacZ mRNAs. In addition, the incorporation of a transcription terminator just downstream of the antisense segment provided for more efficient inhibition of lacZ mRNA translation due to synthesis of smaller and more abundant anti-lacZ mRNAs. The optimal constructions produced undetectable levels of beta-galactosidase synthesis.

Mutations in the Tn10 tet repressor that interfere with induction. Location of the tetracycline-binding domain

Tetracycline induces transcription of the Tn10 tetracycline resistance gene (tetA) by binding to the tet repressor, thereby reducing the repressor's affinity for two operator sites that overlap the tet promoters. We characterized mutations in the tet repressor (tetRs mutations) that interfere with induction of tetA expression. The mutations were isolated on multicopy Tn10 tet plasmids by selecting for resistance to the inducer 5a,6-anhydrotetracycline. Under these conditions, maximal induction of tetA expression inhibits the growth of Escherichia coli K-12. DNA sequence analysis of 25 spontaneous tetRs mutations identified amino acid changes at 13 different positions clustered near the middle of the 207 amino acid residue sequence of tet repressor. This region (residues 64 to 107) is distinct from the bihelical DNA-binding motif of tet repressor (residues 26 to 47). The capacity of tetRs repressors to bind tet operator DNA and to respond to inducer was examined in vivo in tetA-lacZ fusion strains. In three cases, the capacity of tetRs repressors to bind tetracycline was examined in vitro using cell extracts enriched in repressor. Mutations 64Y (His64----Tyr) and 82H (Asn82----His) reduce the repressor's affinity for tetracycline more than 1000-fold and more than 100-fold, respectively, suggesting that His64 and Asn82 may be part of the inducer-binding site or directly involved in maintaining its conformation. Mutation 103I (Thr103----Ile) reduces the repressor's affinity for tetracycline less than tenfold, yet it interferes with induction to a greater extent than either 64Y or 82H, suggesting that 103I may also reduce the repressor's capacity to undergo a conformational change required for induction. The properties of tetRs mutants suggest that the region of amino acid residues 64 to 107 is involved in inducer binding and in signalling between the inducer-binding and operator-binding domains of the repressor.

An N-terminal domain of the tetracycline resistance protein increases susceptibility to aminoglycosides and complements potassium uptake defects in Escherichia coli

Expression of extrachromosomal tet genes increased the susceptibility of gram-negative bacteria to specific aminoglycoside antibiotics. The magnitude of the increase in susceptibility was dependent on the amount and the class of the tet gene product (designated Tet) and the bacterial species in which the tet gene was expressed. Truncated Tet proteins that contained more than the first 33, but not more than the first 97, N-terminal amino acids of Tet also increased the susceptibility to aminoglycosides and complemented the potassium uptake defects in Escherichia coli. The primary structure of this N-terminal Tet fragment has the hydropathic characteristics of a multimeric, transmembrane structure and is highly conserved in three different classes of Tet proteins.

Rapid identification of bacterial genes that are lethal when cloned on multicopy plasmids

A procedure to identify genes that are lethal when cloned on multicopy plasmids was developed. It depends on the ability of mini-Mu plasmid elements to be used for both in vivo cloning and generalized transduction of enterobacterial genes. The feasibility of this procedure was demonstrated by using the tetA gene of Tn10, which is lethal when in multiple copies in the presence of 25 micrograms of tetracycline per ml.

A system for in vivo selection of genomic rearrangements with predetermined endpoints in Escherichia coli using modified Tn10 transposons

Using recombinant DNA techniques, the Tn10-specific tetA gene (coding for tetracycline resistance) has been mutagenized by insertion of a streptomycin-resistance or a kanamycin-resistance gene. The insertions occurred at loci separated by 920 bp. The mutated tetA fragments, respectively designated as Tes (for tetracycline-streptomycin) and Tek (for tetracycline-kanamycin), were subsequently cloned into a phage lambda cIII+cIts857cII+ in replacement of the att lambda region. The two recombinant phages are convenient delivery vehicles which permit the in vivo substitution of the tetA locus of any Tn10 insertion with the Tes or the Tek fragment. The procedure involves two selectable steps: (i) integration of a lambda-Tes (or lambda-Tek) prophage into the Tn10 of interest; (ii) excision of the prophage by a second exchange which leaves the extra resistance gene installed within the Tn10. A major interest of the system is that, once a bacterium carries both Tn10-Tes and Tn10-Tek insertions, a recombination event between the two Tn10 sequences can reconstitute an active tetA gene. This selectable event may be associated with the rearrangement of the sequences surrounding the transposons. This unique property of the "Tes and Tek" system makes it very useful for selection of genomic rearrangements using the Tn10-Tes and Tn10-Tek as predetermined endpoints. The successful isolation of a chromosomal inversion is reported.

Regions associated with the stable maintenance of plasmid pSC101 and its tetracycline resistance

Two regions tentatively called unsA and unsR were identified on pSC101. One, unsA, corresponds to less than 650 bp of the N-terminal in the tetracycline resistance structural gene and seems to inhibit stable maintenance of pSC101. The other, unsR, is defined within the 1 kb XhoI-EcoRI region located upstream of the tetracycline resistance structural gene and is a regulatory gene clearly distinct from tetR (Unger et al. 1984); it serves as a suppressor of the unsA function.

Molecular cloning and expression of hipA, a gene of Escherichia coli K-12 that affects frequency of persistence after inhibition of murein synthesis

The hipA gene at 33.8 min on the Escherichia coli chromosome controls the frequency of persistence upon inhibition of murein synthesis; for strains bearing hipA+ the frequency is 10(-6), and for hipA- strains the frequency is 10(-2). hip+ has been cloned by selection for a kanamycin resistance determinant at 33.9 min. hipA+ is dominant over hipA- in both recA+ and recA- backgrounds. The smallest DNA insert which contains hipA+, as determined by the ability of the plasmids to complement hipA- strains, is 1,885 base pairs. Both orientations of hipA+ are obtained when the cloning site of vector is remote from strong promoters; both orientations complement hipA-, and both encode a unique peptide of 50,000 Mr. The probable direction of transcription has been deduced from the pattern of peptides encoded by plasmids from which either end of the insert and adjacent vector sequences have been deleted. This information and the recovery of only one orientation of hipA+ when the cloning site is close to a strong promoter suggest that a high level of expression of the gene is not tolerated by E. coli.

Effects of toluene permeabilization and cell deenergization on tetracycline resistance in Escherichia coli

Resistance to tetracycline (Tcr) mediated by Tn10 and related Tcr determinants involves an inner membrane protein, TET (similar but not identical for different determinants), and a proton motive force-dependent efflux of tetracycline which keeps the drug away from its intracellular target, the ribosome (L. M. McMurry, R. E. Petrucci, Jr., and S. B. Levy, Proc. Natl. Acad. Sci. USA 77:3974-3977, 1980). However, the amount of tetracycline accumulated by bacteria does not always correlate with their resistance levels, suggesting that an additional resistance mechanism may be present. When we permeabilized susceptible and resistant Tn10-bearing cells with toluene, we found that protein synthesis in the two strains became equally sensitive to tetracycline. Therefore, the protein synthesis machinery was not a source of resistance, and an intact membrane was required for resistance. To determine whether resistance was entirely dependent on energy, we measured susceptibility to tetracycline after inhibition of proton motive force by starvation and specific inhibitors. An 80 to 90% loss of Tcr (measured by protein synthesis) resulted from partial deenergization of resistant cells. A remaining resistance (10- to 20-fold greater than that of susceptible cells) could not be eliminated by further deenergization. These findings indicated that, to a major extent, expression of Tn10 resistance required energy, presumably for tetracycline efflux. They also suggested the existence of a small component of Tcr having little or no energy dependence. Whether this component depends on tetracycline efflux or some other mechanism is not known, but presumably both high- and low-energy components of resistance reflect activity of TET protein.

Promoter mutations affecting divergent transcription in the Tn10 tetracycline resistance determinant

The tetracycline resistance determinant in transposon Tn10 consists of two genes, the tetA resistance gene and the tetR repressor gene, that are transcribed from divergent overlapping promoters. We determined the levels of pulse-labeled tet messenger RNA in Escherichia coli strains with the Tn10 tet genes on a multicopy plasmid. Addition of the inducer 5a,6-anhydrotetracycline results in a 270- to 430-fold increase in tetA mRNA and a 35- to 65-fold increase in tetR mRNA. As judged by the relative molar amounts of tetA and tetR mRNA synthesized under maximally inducing conditions, the tetA promoter (tetPA) is 7 to 11 times more active than the two tetR promoters (tetPR1 and tetPR2) combined. We characterized ten mutations in tetPA, including nine single-base-pair substitutions and a 30-base-pair deletion. All of the single-base-pair changes reduce the agreement with the consensus sequence for promoters recognized by E. coli RNA polymerase. Mutations in highly conserved nucleotides result in a 200- to 600-fold reduction in tetPA activity in vivo. Unexpectedly, tetPA mutations reduce by two- to fourfold the combined activity in vivo of tetPR1 and tetPR2, in spite of their locations outside the -35 and -10 regions of tetPR1 and tetPR2. For two tetPA mutations, the negative effect on tetPR activity was also demonstrated in tetR- tetPR-lacZ operon fusion strains, thus eliminating the possibility that it is due to variations in either plasmid copy-number or induction efficiency. The pleiotropic effects of tetPA mutations are discussed in terms of the expectation that the overlapping tet promoters compete for RNA polymerase.

Resistance to various tetracyclines mediated by transposon Tn10 in Escherichia coli K-12

Levels of resistance to tetracycline, chlortetracycline, demethylchlortetracycline, doxycycline, oxytetracycline, methacycline, pyrrolidinotetracycline, minocycline, and beta-chelocardin of Escherichia coli K-12 carrying transposon Tn10 or defined DNA segments of Tn10 were determined. In all cases, tetA was the only gene required for resistance. Doxycycline was the most effective inducer of tetA gene expression.

Expression of tetracycline resistance in pBR322 derivatives reduces the reproductive fitness of plasmid-containing Escherichia coli

Plasmid pBR322 and its numerous derivatives are used extensively for research and in biotechnology. The tetracycline-resistance (TcR) genes in these plasmids are expressed constitutively and cells carrying these plasmids are resistant to tetracycline. We have shown that expression of the TcR gene has an adverse effect on the reproductive fitness of plasmid-containing bacteria in both glucose-limited batch and chemostat cultures. If the TcR genes are inactivated at any one of three different restriction sites, mixed cultures of plasmid-free and plasmid-containing bacteria grow at the same rate.

New Tn10 derivatives for transposon mutagenesis and for construction of lacZ operon fusions by transposition

We describe below several new variants of the tetracycline-resistance transposon Tn10 which are more useful than the wild-type transposon for many types of genetic and physical analysis of bacteria. These derivatives have one or more of the following new properties: (i) new drug resistance markers; (ii) high transposition frequencies; (iii) removal of the transposase gene to a position outside of the transposing segment; (iv) internal deletions which eliminate the ability of Tn10 to make adjacent deletion/inversions; or (v) addition of a trp-lac operon fusion segment just inside one terminus such that insertion can automatically generate a transcriptional fusion to the interrupted operon. Phage and plasmid vehicles carrying these new elements are described.

Plasmid vectors based on Tn10 DNA: gene expression regulated by tetracycline

The regulatory region of the tetracycline resistance determinant from transposon Tn10 has been used to construct plasmid vectors for gene expression regulated by tetracycline. Plasmids pRS tetBam-8 and pRS tetBam-16 include the tet regulatory region, the segment coding for the first four amino acids of the tetracycline resistance protein (tetA protein), and a linker region with SalI, HpaII, and BamHI restriction sites for gene fusions. Plasmid pTB-1, a derivative of pRS tetBam-8 and of the beta-galactosidase gene-containing plasmid pMC1403, constitutively expresses a tetA fragment-beta-galactosidase fusion protein. If a multicopy runaway replication plasmid, pMOBglII-16 that includes a 2.7-kb BglII DNA fragment from Tnl10 that provides tetR protein is present along with pTB-1, the expression of beta-galactosidase is reduced eightfold. Tetracycline acts as an inducer of the system and restores the level of beta-galactosidase activity measured in transformants containing pTB-1 alone. Plasmid mutants unable to produce active tetR protein are ineffective in reducing expression. Escherichia coli carrying plasmids that express both tetA protein and tetR protein show an increase in the tetracycline resistance level after incubation with the drug. The observations are consistent with the previously proposed mechanism of regulation of tetracycline resistance in Tn10.

Construction of a single-copy promoter vector and its use in analysis of regulation of the transposon Tn10 tetracycline resistance determinant

The construction and characterization of a promoter expression vector, lambda RS205 , is described. lambda RS205 can be used for the in vitro construction of transcriptional (operon) fusions to the lacZ gene of Escherichia coli K-12. The level of beta-galactosidase activity in lysogens of lambda RS205 fusion phages provides a quantitative measure of promoter function under single-copy conditions. The regulation of the Tn10 tetracycline resistance gene ( tetA ) and the Tn10 tet repressor gene (tetR) was examined by inserting DNA fragments that span the tetR- tetA promoter-operator region into lambda RS205 . Levels of beta-galactosidase in tetA -lacZ and tetR-lacZ fusion strains indicate that the tetA and tetR promoters are strong promoters; the tetA promoter is fourfold more active than the tetR promoter. Introduction of tetR+ plasmids into tetA -lacZ and tetR-lacZ fusion strains represses beta-galactosidase synthesis 15- to 60-fold and 6- to 15-fold, respectively. The concentration of tetracycline required to induce half-maximal beta-galactosidase synthesis in these tetR+ tet-lac strains depends on both the tetracycline resistance phenotype and the level of tetR repressor in the fusion strain. However, the induction of beta-galactosidase in isogenic tetA -lacZ and tetR-lacZ strains is coordinate. The data presented here support the current model of Tn10 tet gene organization and regulation and provide quantitative information about the regulation of tetA and tetR in vivo.

Multicopy Tn10 tet plasmids confer sensitivity to induction of tet gene expression

We inserted the Tn10 tetracycline resistance determinant (tet) into the multicopy plasmid pACYC177, and we examined the phenotype of Escherichia coli K-12 strains harboring these plasmids. In agreement with others, we find that Tn10 tet exhibits a negative gene dosage effect. Strains carrying multicopy Tn10 tet plasmids are 4- to 12-fold less resistant to tetracycline than are strains with a single copy of Tn10 in the bacterial chromosome. In addition, we find that multicopy tet strains are 30- to 100-fold less resistant to the tetracycline derivative 5a,6-anhydrotetracycline than are single-copy tet strains. Multicopy tet strains are, in fact, 10- to 25-fold more sensitive to anhydrotetracycline than are strains that lack tet altogether. The hypersensitivity of multi-copy strains to anhydrotetracycline is correlated with the effectiveness of anhydrotetracycline as an inducer of tet gene expression, rather than its effectiveness as an inhibitor of protein synthesis. Anhydrotetracycline is 50- to 100-fold more effective than tetracycline as an inducer of tetracycline resistance and as an inducer of beta-galactosidase in strains that harbor tet-lac gene fusions. In contrast, anhydrotetracycline appears to be two- to fourfold less effective than tetracycline as an inhibitor of protein synthesis. Both anhydrotetracycline and tetracycline induce synthesis of tet polypeptides in minicells harboring multicopy tet plasmids. Differences between E. coli K-12 backgrounds influence the tetracycline and anhydrotetracycline sensitivity of multicopy strains; ZnCl2 enhances the tetracycline and anhydrotetracycline sensitivity of these strains two- to threefold. We propose that the overexpression of one or more Tn10 tet gene products inhibits the growth of multicopy tet strains and accounts for their relative sensitivity to inducers of tet gene expression.