In vivo expression from the RpoS-dependent P1 promoter of the osmotically regulated proU operon in Escherichia coli and Salmonella enterica serovar Typhimurium: activation by rho and hns mutations and by cold stress

Importance of the envelope in Escherichia coli resistance to lithium

The increasing use of lithium (Li) in emerging technologies has prompted concerns about its effects on living microorganisms. To enhance our understanding of the bacterial cytotoxicity of Li, we conducted a deletomic analysis using the bacterial model Escherichia coli. A screen of 3,985 knockout mutants under Li stress highlighted 27 Li-sensitive and 15 Li-resistant mutants. The synthesis of peptidoglycan and the capsule, along with the secretion of colanic acid, contributed to resistance to Li. Ribosomes and the stringent response also seem to play a role in mitigating Li cytotoxicity. A cross-metal comparison revealed that the Li-sensitive phenotype of the mutants was shared with Ca, whereas the resistant phenotype was shared with Mg, Na and K. Moreover, this allowed the identification of ΔacrA as a Li sensitivity-specific mutant. AcrA is a subunit of the AcrAB-TolC efflux pump, which is responsible for the efflux of various xenobiotics. We demonstrate that ΔacrB-ΔtolC accumulates approximately 1.5 times more Li than the WT, indicating that this pump could also facilitate the efflux of Li. This study offers a more comprehensive insight into the mechanisms involved in the Li response in E. coli.

The environmentally-regulated interplay between local three-dimensional chromatin organisation and transcription of proVWX in E. coli

Nucleoid associated proteins (NAPs) maintain the architecture of bacterial chromosomes and regulate gene expression. Thus, their role as transcription factors may involve three-dimensional chromosome re-organisation. While this model is supported by in vitro studies, direct in vivo evidence is lacking. Here, we use RT-qPCR and 3C-qPCR to study the transcriptional and architectural profiles of the H-NS (histone-like nucleoid structuring protein)-regulated, osmoresponsive proVWX operon of Escherichia coli at different osmolarities and provide in vivo evidence for transcription regulation by NAP-mediated chromosome re-modelling in bacteria. By consolidating our in vivo investigations with earlier in vitro and in silico studies that provide mechanistic details of how H-NS re-models DNA in response to osmolarity, we report that activation of proVWX in response to a hyperosmotic shock involves the destabilization of H-NS-mediated bridges anchored between the proVWX downstream and upstream regulatory elements (DRE and URE), and between the DRE and ygaY that lies immediately downstream of proVWX. The re-establishment of these bridges upon adaptation to hyperosmolarity represses the operon. Our results also reveal additional structural features associated with changes in proVWX transcript levels such as the decompaction of local chromatin upstream of the operon, highlighting that further complexity underlies the regulation of this model operon. H-NS and H-NS-like proteins are wide-spread amongst bacteria, suggesting that chromosome re-modelling may be a typical feature of transcriptional control in bacteria.

Mastering the control of the Rho transcription factor for biotechnological applications

The present review represents an update on the fundamental role played by the Rho factor, which facilitates the process of Rho-dependent transcription termination in the prokaryotic world; it also provides a summary of relevant mutations in the Rho factor and the insights they provide into the functions carried out by this protein. Furthermore, a section is dedicated to the putative future use of Rho (the 'taming' of Rho) to facilitate biotechnological processes and adapt them to different technological contexts. Novel bacterial strains can be designed, containing mutations in the rho gene, that are better suited for different biotechnological applications. This process can obtain novel microbial strains that are adapted to lower temperatures of fermentation, shorter production times, exhibit better nutrient utilization, or display other traits that are beneficial in productive Biotechnology. Additional important issues reviewed here include epistasis, the design of TATA boxes, the role of small RNAs, and the manipulation of clathrin-mediated endocytosis, by some pathogenic bacteria, to invade eukaryotic cells. KEY POINTS: • It is postulated that controlling the action of the prokaryotic Rho factor could generate major biotechnological improvements, such as an increase in bacterial productivity or a reduction of the microbial-specific growth rate. • The review also evaluates the putative impact of epistatic mechanisms on Biotechnology, both as possible responsible for unexpected failures in gene cloning and more important for the genesis of new strains for biotechnological applications • The use of clathrin-coated vesicles by intracellular bacterial microorganisms is included too and proposed as a putative delivery mechanism, for drugs and vaccines.

Increased Biomass Production by Mesophilic Food-Associated Bacteria through Lowering the Growth Temperature from 30°C to 10°C

Five isolates from chilled food and refrigerator inner surfaces and closely related reference strains of the species Escherichia coli, Listeria monocytogenes, Staphylococcus xylosus, Bacillus cereus, Pedobacter nutrimenti, and Pedobacter panaciterrae were tested for the effect of growth temperature (30°C and 10°C) on biomass formation. Growth was monitored via optical density, and biomass formation was measured at the early stationary phase based on the following parameters in complex and defined media: viable cell count, total cell count, cell dry weight, whole-cell protein content, and cell morphology. According to the lack of growth at 1°C, all strains were assigned to the thermal class of mesophiles. Glucose and ammonium consumption related to cell yield were analyzed in defined media. Except for the protein content, temperature had a significant (t test, P < 0.05) effect on all biomass formation parameters for each strain. The results show a significant difference between the isolates and the related reference strains. Isolates achieved an increase in biomass production between 20% and 110% at the 10°C temperature, which is 15 to 25°C lower than their maximum growth rate temperatures. In contrast, reference strains showed a maximum increase of only about 25%, and some reference strains showed no increase or a decrease of approximately 25%. As expected, growth rates for all strains were higher at 30°C than at 10°C, while biomass production for isolates was higher at 10°C than at 30°C. In contrast, the reference strains showed similar growth yields at the two temperatures. This also demonstrates for mesophilic bacterial strains more efficient nutrient assimilation during growth at low temperatures. Until now, this characteristic was attributed only to psychrophilic microorganisms.

IMPORTANCE

For several psychrophilic species, increased biomass formation was described at temperatures lower than optimum growth temperatures, which are defined by the highest growth rate. This work shows increased biomass formation at low growth temperatures for mesophilic isolates. A comparison with closely related reference strains from culture collections showed a significantly smaller increase or no increase in biomass formation. This indicates a loss of specific adaptive mechanisms (e.g., cold adaptation) for mesophiles during long-term cultivation. The increased biomass production for mesophiles under low-temperature conditions opens new avenues for a more efficient biotechnological transformation of nutrients to microbial biomass. These findings may also be important for risk assessment of cooled foods since risk potential is often correlated with the cell numbers present in food samples.

Osmotolerance provided by the alternative sigma factors σB and rpoS to Staphylococcus aureus and Escherichia coli is solute dependent and does not result in an increased growth fitness in NaCl containing media

The aim of this work was to examine the role of the alternative general stress sigma factors σ(B) and rpoS on the ability of Staphylococcus aureus and Escherichia coli, respectively, to grow in liquid and solid media of different osmolarity. For this purpose, S. aureus strain Newman and its isogenic ΔsigB mutant IK84 and E. coli strain BJ4 and its isogenic ΔrpoS mutant BJ4L1 were grown in media (TSBYE) with different concentrations of NaCl. Growth parameters (lag phase duration, growth rate and maximum number of microorganisms) and limiting growth concentrations (Maximum Non-Inhibitory Concentration - MNIC - and Minimum Inhibitory Concentration - MIC-) were determined. The mechanisms underlying the differences observed between parental and mutant strains were also explored. The absence of the sigma factors σ(B) and rpoS led to a decrease in the MNICs and MICs calculated for S. aureus and E. coli, respectively. Conversely, neither σ(B) nor rpoS provided with increased growth fitness to S. aureus and E. coli cells at NaCl concentrations up to 1.36M and 1M, respectively. The decreased osmotolerance of the σ(B) and rpoS deficient strains, as compared to their parental strains, was compensated by the addition of glycine-betaine (1mM) to the growth medium. It was also observed that the decreased tolerance to NaCl of the mutant strains was coincident with a decreased tolerance to sucrose, KCl, and LiCl but not to glycerol, MgCl2, and CaCl2. Results obtained also demonstrate that the increased osmotolerance of stationary growth phase E. coli cells, as compared to exponential growth phase ones, would be due to the activation of both rpoS-independent and rpoS-dependent mechanisms. This work will help to understand the mechanisms of bacterial resistance to osmotic stress and the role of the alternative sigma factors σ(B) and rpoS in this process.

Silencing by H-NS potentiated the evolution of Salmonella

The bacterial H-NS protein silences expression from sequences with higher AT-content than the host genome and is believed to buffer the fitness consequences associated with foreign gene acquisition. Loss of H-NS results in severe growth defects in Salmonella, but the underlying reasons were unclear. An experimental evolution approach was employed to determine which secondary mutations could compensate for the loss of H-NS in Salmonella. Six independently derived S. Typhimurium hns mutant strains were serially passaged for 300 generations prior to whole genome sequencing. Growth rates of all lineages dramatically improved during the course of the experiment. Each of the hns mutant lineages acquired missense mutations in the gene encoding the H-NS paralog StpA encoding a poorly understood H-NS paralog, while 5 of the mutant lineages acquired deletions in the genes encoding the Salmonella Pathogenicity Island-1 (SPI-1) Type 3 secretion system critical to invoke inflammation. We further demonstrate that SPI-1 misregulation is a primary contributor to the decreased fitness in Salmonella hns mutants. Three of the lineages acquired additional loss of function mutations in the PhoPQ virulence regulatory system. Similarly passaged wild type Salmonella lineages did not acquire these mutations. The stpA missense mutations arose in the oligomerization domain and generated proteins that could compensate for the loss of H-NS to varying degrees. StpA variants most able to functionally substitute for H-NS displayed altered DNA binding and oligomerization properties that resembled those of H-NS. These findings indicate that H-NS was central to the evolution of the Salmonellae by buffering the negative fitness consequences caused by the secretion system that is the defining characteristic of the species.

The Sm-like RNA chaperone Hfq mediates transcription antitermination at Rho-dependent terminators

In Escherichia coli, the essential motor protein Rho promotes transcription termination in a tightly controlled manner that is not fully understood. Here, we show that the general post-transcriptional regulatory protein Hfq associates with Rho to regulate Rho function. The Hfq:Rho complex can be further stabilized by RNA bridging both factors in a configuration that inhibits the ATP hydrolysis and duplex unwinding activities of Rho and that mediates transcription antitermination at Rho-dependent terminators in vitro and in vivo. Antitermination at a prototypical terminator (λtR1) requires Hfq binding to an A/U-rich transcript region directly upstream from the terminator. Antitermination is modulated by trans-acting factors (NusG or nucleic acid competitors) that affect Hfq association with Rho or RNA. These data unveil a new Hfq function and a novel transcription regulatory mechanism with potentially important implications for bacterial RNA metabolism, gene silencing, and pathogenicity.

Testing temperature-induced proteomic changes in the plant-associated bacterium Pseudomonas fluorescens SBW25

Traits used by bacteria to enhance ecological performance in natural environments are not well understood. Recognizing that the saprophytic plant-colonizing bacterium Pseudomonas fluorescens SBW25 experiences temperatures in its natural environment significantly cooler than the 28°C routinely used in the laboratory, we identified proteins differentially expressed between 28°C and the more environmentally relevant temperature of 14°C. Of 2102 protein isoforms, 32 were temperature responsive and identified by mass spectrometry. Seven of these (OmpR, MucD, GuaD, OsmY and three of unknown function, Tee1, Tee2 and Tee3) were selected for genetic and ecological analyses. In each instance, changes in protein expression with temperature were mirrored by parallel transcriptional changes. The fitness contribution of the genes encoding each of the seven proteins was larger at 14°C than 28°C and included two cases of trade-offs (enhanced fitness at one temperature and reduced fitness at the other -mucD and tee2 deletions). The relationship between the fitness effects of genes in vitro and in vivo was variable, but two temperature-responsive genes -osmY and mucD- contribute substantially to the ability of P. fluorescens to colonize the plant environment.

Mycobacterium tuberculosis nucleoid-associated DNA-binding protein H-NS binds with high-affinity to the Holliday junction and inhibits strand exchange promoted by RecA protein

A number of studies have shown that the structure and composition of bacterial nucleoid influences many a processes related to DNA metabolism. The nucleoid-associated proteins modulate not only the DNA conformation but also regulate the DNA metabolic processes such as replication, recombination, repair and transcription. Understanding of how these processes occur in the context of Mycobacterium tuberculosis nucleoid is of considerable medical importance because the nucleoid structure may be constantly remodeled in response to environmental signals and/or growth conditions. Many studies have concluded that Escherichia coli H-NS binds to DNA in a sequence-independent manner, with a preference for A-/T-rich tracts in curved DNA; however, recent studies have identified the existence of medium- and low-affinity binding sites in the vicinity of the curved DNA. Here, we show that the M. tuberculosis H-NS protein binds in a more structure-specific manner to DNA replication and repair intermediates, but displays lower affinity for double-stranded DNA with relatively higher GC content. Notably, M. tuberculosis H-NS was able to bind Holliday junction (HJ), the central recombination intermediate, with substantially higher affinity and inhibited the three-strand exchange promoted by its cognate RecA. Likewise, E. coli H-NS was able to bind the HJ and suppress DNA strand exchange promoted by E. coli RecA, although much less efficiently compared to M. tuberculosis H-NS. Our results provide new insights into a previously unrecognized function of H-NS protein, with implications for blocking the genome integration of horizontally transferred genes by homologous and/or homeologous recombination.

Examination of stress and virulence gene expression in Escherichia coli O157:H7 using targeted microarray analysis

Escherichia coli O157:H7 poses a threat to humans through food- and water-borne transmission. To investigate how environmental stresses affect the Escherichia coli O157:H7 transcriptome, we designed a targeted microarray consisting of stress response and virulence genes (n = 125) to analyze the impact of acidified (pH 3.5), cold (7.5 degrees C), and fresh tryptic soy broth (TSB) (37 degrees C) on E. coli O157:H7 stress response and virulence gene expression. Nutrient replenishment with fresh TSB resulted in 72 differentially expressed genes (> or = 1.5-fold change; p < 0.05), with 65 induced. All queried global and specific stress regulators were affected, as were 12 virulence genes. Cold-shocked cells displayed 17 differentially expressed genes, with 10 being induced. Induction of rpoS, members of the sigma(H) regulon (clpB, dnaK, ftsH), and acid resistance (AR) genes (gadA, gadX) was observed. Porin transcript (ompC, ompF) and gapA and tufA ancillary genes were repressed. Acid shock resulted in 24 differentially expressed genes, with 21 induced. No induction of any stationary phase AR system was observed, though acid-coping mechanisms were recruited, including mar and phoB induction, and repression of ompC and ompF. Stress regulators were induced, including relA, soxS, rpoE, and rpoH. The microarray data were validated by quantitative real-time polymerase chain reaction. Exposure to sublethal stress events led to the induction of diverse stress response networks. In the food chain, sublethal events may render cells increasingly resistant to future stresses, potentially leading to increased survival.

Environmental regulation operating at the promoter clearance step of bacterial transcription

In vivo transcription of the Escherichia coli argO gene, which encodes an arginine (Arg) exporter, requires the LysR-family regulator protein ArgP (previously called IciA) and is induced in the presence of Arg or its naturally occurring antimetabolite analog canavanine. Lysine (Lys) addition, on the other hand, phenocopies an argP mutation to result in the shutoff of argO expression. We now report that the ArgP dimer by itself is able to bind the argO promoter-operator region to form a binary complex, but that the formation of a ternary complex with RNA polymerase is greatly stimulated only in presence of a coeffector. Both Arg and Lys were proficient as coeffectors for ArgP-mediated recruitment of RNA polymerase to, and open complex formation at, the argO promoter, although only Arg (but not Lys) was competent to activate transcription. The two coeffectors competed for binding to ArgP, and the ternary complex that had been assembled on the argO template in the presence of Lys could be chased into a transcriptionally active state upon Arg addition. Our results support a novel mechanism of argO regulation in which Lys-bound ArgP reversibly restrains RNA polymerase at the promoter, at a step (following open complex formation) that precedes, and is common to, both abortive and productive transcription. This represents, therefore, the first example of an environmental signal regulating the final step of promoter clearance by RNA polymerase in bacterial transcription. We propose that, in E. coli cells, the ternary complex remains assembled and poised at the argO promoter at all times to respond, positively or negatively, to instantaneous changes in the ratio of intracellular Arg to Lys concentrations.

Silencing of xenogeneic DNA by H-NS--facilitation of lateral gene transfer in bacteria by a defense system that recognizes foreign DNA

Lateral gene transfer has played a prominent role in bacterial evolution, but the mechanisms allowing bacteria to tolerate the acquisition of foreign DNA have been incompletely defined. Recent studies show that H-NS, an abundant nucleoid-associated protein in enteric bacteria and related species, can recognize and selectively silence the expression of foreign DNA with higher adenine and thymine content relative to the resident genome, a property that has made this molecule an almost universal regulator of virulence determinants in enteric bacteria. These and other recent findings challenge the ideas that curvature is the primary determinant recognized by H-NS and that activation of H-NS-silenced genes in response to environmental conditions occurs through a change in the structure of H-NS itself. Derepression of H-NS-silenced genes can occur at specific promoters by several mechanisms including competition with sequence-specific DNA-binding proteins, thereby enabling the regulated expression of foreign genes. The possibility that microorganisms maintain and exploit their characteristic genomic GC ratios for the purpose of self/non-self-discrimination is discussed.

H-NS cooperative binding to high-affinity sites in a regulatory element results in transcriptional silencing

H-NS is a protein of the bacterial nucleoid involved in DNA compaction and transcription regulation. In vivo, H-NS selectively silences specific genes of the bacterial chromosome. However, many studies have concluded that H-NS binds sequence-independently to DNA, leaving the molecular basis for its selectivity unexplained. We show that the negative regulatory element (NRE) of the supercoiling-sensitive Escherichia coliproU gene contains two identical high-affinity binding sites for H-NS. Cooperative binding of H-NS is abrogated by changes in DNA superhelical density and temperature. We further demonstrate that the high-affinity sites nucleate cooperative binding and establish a nucleoprotein structure required for silencing. Mutations in these sites result in loss of repression by H-NS. In this model, silencing at proU, and by inference at other genes directly regulated by H-NS, is tightly controlled by the cooperativity between bound H-NS molecules.

Reciprocal transcriptional and posttranscriptional growth-phase-dependent expression of sfh, a gene that encodes a paralogue of the nucleoid-associated protein H-NS

The IncHI1 self-transmissible plasmid pSf-R27 from Shigella flexneri 2a strain 2457T harbors sfh, a gene that codes for a protein with strong amino acid sequence homology to the global transcription regulator and nucleoid-associated protein H-NS and to its paralogue, StpA. Previously, we discovered that the expression of sfh mRNA is growth phase dependent such that in cultures growing in Lennox broth at 37 degrees C, the transcript is readily detectable in the early stages of exponential growth but is not detectable at the onset of stationary phase. In contrast, the Sfh protein is poorly expressed in early-exponential growth when sfh mRNA is abundant whereas it is expressed to a high level in early stationary phase, when sfh transcript expression is low (P. Deighan, C. Beloin, and C. J. Dorman, Mol. Microbiol. 48:1401-1416, 2003). This unusual pattern of reciprocal mRNA and protein expression is not due to growth phase-dependent effects on either mRNA or protein stability, nor is it due to the known abilities of the Sfh, StpA, and H-NS proteins to influence sfh gene expression. Instead, our data point to a blockade of sfh mRNA translation in early-exponential growth that is relieved as the culture enters the stationary phase of growth. Replacing the 5' end and translation initiation signals of the sfh mRNA with heterologous sequences did not alter the growth phase-dependent expression of the Sfh protein, suggesting that growth phase control of translation is intrinsic to another component of the message.

Host factor titration by chromosomal R-loops as a mechanism for runaway plasmid replication in transcription termination-defective mutants of Escherichia coli

Two Escherichia coli genes, rnhA and recG, encode products that disrupt R-loops by hydrolysis and unwinding, respectively. It is known that the propensity for R-loop formation in vivo is increased during growth at 21 degrees C. We have identified several links between rnhA, recG, and R-loop-dependent plasmid replication on the one hand, and genes rho and nusG involved in factor-dependent transcription termination on the other. A novel nusG-G146D mutation phenocopied a rho-A243E mutation in conferring global deficiency in transcription termination, and both mutants were killed at 21 degrees C following overexpression of rnhA(+). Mutant combinations rnhA-nusG or recG-rho were synthetically lethal at 21 degrees C, with the former being suppressed by recG(+) overexpression. rho and nusG mutants were killed following transformation with plasmids such as pACYC184 or pUC19 (which have R-loop replication intermediates) even at 30 degrees C or 37 degrees C, and the lethality was correlated with greatly increased content of supercoiled monomer species of these and other co-resident R-loop-dependent plasmids. Plasmid-mediated lethality in the mutants was suppressed by overexpression of rnhA(+) or recG(+). Two additional categories of trans-acting suppressors of the plasmid-mediated lethality were identified whose primary effects were, respectively, a reduction in plasmid copy number even in the wild-type strain, and a restoration of the proficiency of in vivo transcription termination in the nusG and rho mutant strains. The former category of suppressors included rom(+), and mutations in rpoB(Q513L), pcnB, and polA, whereas the latter included a mutation in rho (R221C) and several non-null mutations (E74K, L26P, and delta64-137) in the gene encoding the nucleoid protein H-NS. We propose that an increased occurrence of chromosomal R-loops in the rho and nusG mutants leads to titration of a cyloplasmic host factor(s) that negatively modulates the stability of plasmid R-loop replication intermediates and consequently to runaway plasmid replication.

The degree of oligomerization of the H-NS nucleoid structuring protein is related to specific binding to DNA

At several E. coli promoters, initiation of transcription is repressed by a tight nucleoprotein complex formed by the assembly of the H-NS protein. In order to characterize the relationship between the structure of H-NS oligomers in solution and on relevant DNA fragments, we have compared wild-type H-NS and several transdominant H-NS mutants using gel shift assays, DNase I footprinting, analytical ultracentrifugation, and reactivity toward a cross-linking reagent. In solution, oligomerization occurs through two protein interfaces, one necessary to construct a dimeric core (and involving residues 1-64) and the other required for subsequent assembly of these dimers. We show that, as well as region 64-95, residues present in the NH(2)-terminal coiled coil domain also participate in this second interface. Our results support the view that the same interacting interfaces are also involved on the DNA. We propose that the dimeric core recognizes specific motifs, with the second interface being critical for their correct head to tail assembly. The COOH-terminal domain of the protein contains the DNA binding motif essential for the discrimination of this specific functional assembly over competitive nonspecific H-NS polymers.

An N-terminally truncated RpoS (sigma(S)) protein in Escherichia coli is active in vivo and exhibits normal environmental regulation even in the absence of rpoS transcriptional and translational control signals

RpoS (sigma(S)) in Escherichia coli is a stationary-phase-specific primary sigma factor of RNA polymerase which is 330 amino acids long and belongs to the eubacterial sigma(70) family of proteins. Conserved domain 1.1 at the N-terminal end of sigma(70) has been shown to be essential for RNA polymerase function, and its deletion has been shown to result in a dominant-lethal phenotype. We now report that a sigma(S) variant with a deletion of its N-terminal 50 amino acids (sigma(S)Delta1-50), when expressed in vivo either from a chromosomal rpoS::IS10 allele (in rho mutant strains) or from a plasmid-borne arabinose-inducible promoter, is as proficient as the wild type in directing transcription from the proU P1 promoter; at three other sigma(S)-dependent promoters that were tested (osmY, katE, and csiD), the truncated protein exhibited a three- to sevenfold reduced range of activities. Catabolite repression at the csiD promoter (which requires both sigma(S) and cyclic AMP [cAMP]-cAMP receptor protein for its activity) was also preserved in the strain expressing sigma(S)Delta1-50. The intracellular content of sigma(S)Delta1-50 was regulated by culture variables such as growth phase, osmolarity, and temperature in the same manner as that described earlier for sigma(S), even when the truncated protein was expressed from a template that possessed neither the transcriptional nor the translational control elements of wild-type rpoS. Our results indicate that, unlike that in sigma(70), the N-terminal domain in sigma(S) may not be essential for the protein to function as a sigma factor in vivo. Furthermore, our results suggest that the induction of sigma(S)-specific promoters in stationary phase and during growth under conditions of high osmolarity or low temperature is mediated primarily through the regulation of sigma(S) protein degradation.

The transcription termination factor Rho is required for oxidative stress survival in Caulobacter crescentus

A transposon Tn5 mutagenesis library was generated from Caulobacter crescentus strain NA1000, and clones with deficiency in survival in a high concentration of NaCl were selected. One of these clones, 37G10, has the Tn5 integrated within the coding region of the transcription termination factor Rho. Analysis of this mutant phenotype showed that the cells are motile and present a normal cell cycle, but have a longer generation time. This strain is sensitive to acidic pH, to the presence of different salts and to heat shock, but it responds well to UV light and alkaline pH. The most striking phenotype of the rho mutant is that it is extremely sensitive to oxidative stress, in both exponential and stationary phases. Experiments using a transcriptional fusion of the rho promoter region to the lacZ gene showed that rho gene expression varies during the cell cycle, showing very low expression levels at the swarmer cell stage and presenting maximum levels in early predivisional cells. Transcription of the rho gene is increased in the rho mutant strain, which is indicative of an autoregulatory circuit, and there is a small variation in the cell cycle pattern of expression. Several peptides have their synthesis altered in the mutant strain, as analysed by two-dimensional gel electrophoresis, most of which show a reduction in expression. These results indicate that the Rho factor is essential for an efficient response to certain stresses in Caulobacter.