The DEAD-box RNA helicase RhlB is required for efficient RNA processing at low temperature in Caulobacter

IMPORTANCE

One of the most important control points in gene regulation is RNA stability, which determines the half-life of a transcript from its transcription until its degradation. Bacteria have evolved a sophisticated multi-enzymatic complex, the RNA degradosome, which is dedicated mostly to RNA turnover. The combined activity of RNase E and the other RNA degradosome enzymes provides an efficient pipeline for the complete degradation of RNAs. The DEAD-box RNA helicases are very often found in RNA degradosomes from phylogenetically distant bacteria, confirming their importance in unwinding structured RNA for subsequent degradation. This work showed that the absence of the RNA helicase RhlB in the free-living Alphaproteobacterium Caulobacter crescentus causes important changes in gene expression and cell physiology. These are probably due, at least in part, to inefficient RNA processing by the RNA degradosome, particularly at low-temperature conditions.

Cold Regulation of Genes Encoding Ion Transport Systems in the Oligotrophic Bacterium Caulobacter crescentus

In this study, we characterize the response of the free-living oligotrophic alphaproteobacterium Caulobacter crescentus to low temperatures by global transcriptomic analysis. Our results showed that 656 genes were upregulated and 619 were downregulated at least 2-fold after a temperature downshift. The identified differentially expressed genes (DEG) belong to several functional categories, notably inorganic ion transport and metabolism, and a subset of these genes had their expression confirmed by reverse transcription quantitative real-time PCR (RT-qPCR). Several genes belonging to the ferric uptake regulator (Fur) regulon were downregulated, indicating that iron homeostasis is relevant for adaptation to cold. Several upregulated genes encode proteins that interact with nucleic acids, particularly RNA: cspA, cspB, and the DEAD box RNA helicases rhlE, dbpA, and rhlB. Moreover, 31 small regulatory RNAs (sRNAs), including the cell cycle-regulated noncoding RNA (ncRNA) CcnA, were upregulated, indicating that posttranscriptional regulation is important for the cold stress response. Interestingly, several genes related to transport were upregulated under cold stress, including three AcrB-like cation/multidrug efflux pumps, the nitrate/nitrite transport system, and the potassium transport genes kdpFABC. Further characterization showed that kdpA is upregulated in a potassium-limited medium and at a low temperature in a SigT-independent way. kdpA mRNA is less stable in rho and rhlE mutant strains, but while the expression is positively regulated by RhlE, it is negatively regulated by Rho. A kdpA-deleted strain was generated, and its viability in response to osmotic, acidic, or cold stresses was determined. The implications of such variation in the gene expression for cold adaptation are discussed. IMPORTANCE Low-temperature stress is an important factor for nucleic acid stability and must be circumvented in order to maintain the basic cell processes, such as transcription and translation. The oligotrophic lifestyle presents further challenges to ensure the proper nutrient uptake and osmotic balance in an environment of slow nutrient flow. Here, we show that in Caulobacter crescentus, the expression of the genes involved in cation transport and homeostasis is altered in response to cold, which could lead to a decrease in iron uptake and an increase in nitrogen and high-affinity potassium transport by the Kdp system. This previously uncharacterized regulation of the Kdp transporter has revealed a new mechanism for adaptation to low temperatures that may be relevant for oligotrophic bacteria.

Protein Pulldown Assays to Monitor the Composition of the Bacterial RNA Degradosome

The method of co-immunoprecipitation (co-IP or pulldown) enables the identification of proteins interacting in macromolecular assemblies, through the purification of a key protein by affinity chromatography using specific antibodies immobilized on a matrix. The advantages of using epitope-tagged proteins include the ability to use commercially available antibodies for affinity purifications, and typically they do not disrupt the structure of the protein complexes. Here we describe the utilization of an epitope-tagged version of Caulobacter crescentus RNase E in order to determine the composition of the RNA degradosome under different growth conditions. Several proteins that interact with the RNA degradosome were identified.

Identification of Hfq-binding RNAs in Caulobacter crescentus

Small RNAs are important for post-transcriptional regulation of gene expression, affecting stability and activity of their target mRNAs. The bacterial Sm-like protein Hfq is required to promote pairing between both RNAs when their sequence complementarity is limited. To provide a first global view on the post-transcriptional landscape of the α-proteobacterium Caulobacter crescentus, we have identified the Hfq-binding RNAs employing High-throughput sequencing of RNA isolated by cross-linking immunoprecipitation (HITS-CLIP). A total of 261 RNAs, including 3 unannotated RNAs, were successfully identified and classified according to putative function. Moreover, possible interactions between the identified sRNAs with mRNA targets were postulated through computational target predictions.

Iron Deficiency Generates Oxidative Stress and Activation of the SOS Response in Caulobacter crescentus

In C. crescentus, iron metabolism is mainly controlled by the transcription factor Fur (ferric uptake regulator). Iron-bound Fur represses genes related to iron uptake and can directly activate the expression of genes for iron-containing proteins. In this work, we used total RNA sequencing (RNA-seq) of wild type C. crescentus growing in minimal medium under iron limitation and a fur mutant strain to expand the known Fur regulon, and to identify novel iron-regulated genes. The RNA-seq of cultures treated with the iron chelator 2-2-dypiridyl (DP) allowed identifying 256 upregulated genes and 236 downregulated genes, being 176 and 204 newly identified, respectively. Sixteen transcription factors and seven sRNAs were upregulated in iron limitation, suggesting that the response to low iron triggers a complex regulatory network. Notably, lexA along with most of its target genes were upregulated, suggesting that DP treatment caused DNA damage, and the SOS DNA repair response was activated in a RecA-dependent manner, as confirmed by RT-qPCR. Fluorescence microscopy assays using an oxidation-sensitive dye showed that wild type cells in iron limitation and the fur mutant were under endogenous oxidative stress, and a direct measurement of cellular H₂O₂ showed that cells in iron-limited media present a higher amount of endogenous H₂O₂. A mutagenesis assay using the rpoB gene as a reporter showed that iron limitation led to an increase in the mutagenesis rate. These results showed that iron deficiency causes C. crescentus cells to suffer oxidative stress and to activate the SOS response, indicating an increase in DNA damage.

Molecular characterization of Caulobacter crescentus mutator strains

Mutator strains were identified by screening random Tn5 insertion clones of Caulobacter crescentus. We identified clones with robust increases in mutation rates with Tn5 insertions in the mutY, mutS, mutL and uvrD genes, known to act in mutation-preventing pathways in Escherichia coli. Analysis of mutations in the rpoB gene revealed that in both the parental strain and mismatch repair-deficient mutants, A:T→G:C transitions predominate by a large margin over C:G→T:A. We have also investigated the role of the error-prone polymerase encoded by imuC (dnaE2) in spontaneous mutagenesis, and found that a imuC mutant strain shows mutation rates and sequences comparable to the parental strain. Our study characterizes for the first time mutator strains in a member of the alphaproteobacteria group. In spite of the limitations of using a single marker, possible reasons for the observed mutational bias are discussed in the light of the repertoire of DNA repair genes in this bacterium.

Transcriptomic analysis of the stationary phase response regulator SpdR in Caulobacter crescentus

Background

As bacterial cells enter stationary phase, they adjust their growth rate to comply with nutrient restriction and acquire increased resistance to several stresses. These events are regulated by controlling gene expression at this phase, changing the mode of exponential growth into that of growth arrest, and increasing the expression of proteins involved in stress resistance. The two-component system SpdR/SpdS is required for the activation of transcription of the Caulobacter crescentus cspD gene at the onset of stationary phase.

Results

In this work, we showed that both SpdR and SpdS are also induced upon entry into stationary phase, and this induction is partly mediated by ppGpp and it is not auto-regulated. Global transcriptional analysis at early stationary phase of a spdR null mutant strain compared to the wild type strain was carried out by DNA microarray. Twenty-three genes showed at least twofold decreased expression in the spdR deletion mutant strain relative to its parental strain, including cspD, while five genes showed increased expression in the mutant. The expression of a set of nine genes was evaluated by quantitative real time PCR, validating the microarray data, and indicating an important role for SpdR at stationary phase. Several of the differentially expressed genes can be involved in modulating gene expression, including four transcriptional regulators, and the RNA regulatory protein Hfq. The ribosomal proteins NusE and NusG, which also have additional regulatory functions in transcription and translation, were also downregulated in the spdR mutant, as well as the ParE1 toxin. The purified SpdR protein was shown to bind to the regulatory region of CC0517 by Electrophoretic Mobility Shift Assay, and the SpdR-regulated gene CC0731 was shown to be expressed at a lower level in the null cspD mutant, suggesting that at least part of the effect of SpdR on the expression of this gene is indirect.

Conclusions

The results indicate that SpdR regulates several genes encoding proteins of regulatory function, which in turn may be required for the expression of other genes important for the transition to stationary phase.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-016-0682-y) contains supplementary material, which is available to authorized users.

CspC regulates the expression of the glyoxylate cycle genes at stationary phase in Caulobacter

Background

The Cold Shock proteins are RNA binding proteins involved in various cellular processes, including adaptation to low temperature, nutritional stress, cell growth and stationary phase. They may have an impact on gene expression by interfering with RNA stability and acting as transcription antiterminators. Caulobacter crescentus cspC is an essential gene encoding a stationary phase-induced protein of the Cold Shock Protein family and this work had as goal investigating the basis for the requirement of this gene for survival at this phase. In this work we investigate the role of CspC in C. crescentus stationary phase and discuss the molecular mechanisms that could be involved.

Results

The expression of cspC increased significantly at stationary phase in complex media and in glucose depletion, indicating a putative role in responding to carbon starvation. Global transcriptional profiling experiments comparing cspC and the wild type strain both at exponential and stationary phases as well as comparing exponential and stationary phase in wild type strain were carried out by DNA microarray analysis. The results showed that the absence of cspC affected the transcription of 11 genes at exponential phase and 60 genes at stationary phase. Among the differentially expressed genes it is worth noting those encoding respiratory enzymes and genes for sulfur metabolism, which were upregulated, and those encoding enzymes of the glyoxylate cycle, which were severely downregulated in the mutant at stationary phase. mRNA decay experiments showed that the aceA mRNA, encoding isocitrate lyase, was less stable in the cspC mutant, indicating that this effect was at least partially due to posttranscriptional regulation. These observations were supported by the observed arrested growth phenotype of the cspC strain when grown in acetate as the sole carbon source, and by the upregulation of genes for assimilatory sulfate reduction and methionine biosynthesis.

Conclusions

The stationary phase-induced RNA binding protein CspC has an important role in gene expression at this phase, and is necessary for maximal expression of the glyoxylate cycle genes. In the case of aceA, its downregulation may be attributed to the shorter half-life of the mRNA in the cspC mutant, indicating that one of the possible regulatory mechanisms is via altering RNA stabilization.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1845-1) contains supplementary material, which is available to authorized users.

Leptospira interrogans serovar copenhageni harbors two lexA genes involved in SOS response

Bacteria activate a regulatory network in response to the challenges imposed by DNA damage to genetic material, known as the SOS response. This system is regulated by the RecA recombinase and by the transcriptional repressor lexA. Leptospira interrogans is a pathogen capable of surviving in the environment for weeks, being exposed to a great variety of stress agents and yet retaining its ability to infect the host. This study aims to investigate the behavior of L. interrogans serovar Copenhageni after the stress induced by DNA damage. We show that L. interrogans serovar Copenhageni genome contains two genes encoding putative LexA proteins (lexA1 and lexA2) one of them being potentially acquired by lateral gene transfer. Both genes are induced after DNA damage, but the steady state levels of both LexA proteins drop, probably due to auto-proteolytic activity triggered in this condition. In addition, seven other genes were up-regulated following UV-C irradiation, recA, recN, dinP, and four genes encoding hypothetical proteins. This set of genes is potentially regulated by LexA1, as it showed binding to their promoter regions. All these regions contain degenerated sequences in relation to the previously described SOS box, TTTGN ₅CAAA. On the other hand, LexA2 was able to bind to the palindrome TTGTAN₁₀TACAA, found in its own promoter region, but not in the others. Therefore, the L. interrogans serovar Copenhageni SOS regulon may be even more complex, as a result of LexA1 and LexA2 binding to divergent motifs. New possibilities for DNA damage response in Leptospira are expected, with potential influence in other biological responses such as virulence.

Global transcriptional response of Caulobacter crescentus to iron availability

Background

In the alpha subclass of proteobacteria iron homeostasis is controlled by diverse iron responsive regulators. Caulobacter crescentus, an important freshwater α-proteobacterium, uses the ferric uptake repressor (Fur) for such purpose. However, the impact of the iron availability on the C. crescentus transcriptome and an overall perspective of the regulatory networks involved remain unknown.

Results

In this work we report the identification of iron-responsive and Fur-regulated genes in C. crescentus using microarray-based global transcriptional analyses. We identified 42 genes that were strongly upregulated both by mutation of fur and by iron limitation condition. Among them, there are genes involved in iron uptake (four TonB-dependent receptor gene clusters, and feoAB), riboflavin biosynthesis and genes encoding hypothetical proteins. Most of these genes are associated with predicted Fur binding sites, implicating them as direct targets of Fur-mediated repression. These data were validated by β-galactosidase and EMSA assays for two operons encoding putative transporters. The role of Fur as a positive regulator is also evident, given that 27 genes were downregulated both by mutation of fur and under low-iron condition. As expected, this group includes many genes involved in energy metabolism, mostly iron-using enzymes. Surprisingly, included in this group are also TonB-dependent receptors genes and the genes fixK, fixT and ftrB encoding an oxygen signaling network required for growth during hypoxia. Bioinformatics analyses suggest that positive regulation by Fur is mainly indirect. In addition to the Fur modulon, iron limitation altered expression of 113 more genes, including induction of genes involved in Fe-S cluster assembly, oxidative stress and heat shock response, as well as repression of genes implicated in amino acid metabolism, chemotaxis and motility.

Conclusions

Using a global transcriptional approach, we determined the C. crescentus iron stimulon. Many but not all of iron responsive genes were directly or indirectly controlled by Fur. The iron limitation stimulon overlaps with other regulatory systems, such as the RpoH and FixK regulons. Altogether, our results showed that adaptation of C. crescentus to iron limitation not only involves increasing the transcription of iron-acquisition systems and decreasing the production of iron-using proteins, but also includes novel genes and regulatory mechanisms.

Two RND proteins involved in heavy metal efflux in Caulobacter crescentus belong to separate clusters within proteobacteria

Background

Heavy metal Resistance-Nodulation-Division (HME-RND) efflux systems help Gram-negative bacteria to keep the intracellular homeostasis under high metal concentrations. These proteins constitute the cytoplasmic membrane channel of the tripartite RND transport systems. Caulobacter crescentus NA1000 possess two HME-RND proteins, and the aim of this work was to determine their involvement in the response to cadmium, zinc, cobalt and nickel, and to analyze the phylogenetic distribution and characteristic signatures of orthologs of these two proteins.

Results

Expression assays of the czrCBA operon showed significant induction in the presence of cadmium and zinc, and moderate induction by cobalt and nickel. The nczCBA operon is highly induced in the presence of nickel and cobalt, moderately induced by zinc and not induced by cadmium. Analysis of the resistance phenotype of mutant strains showed that the ΔczrA strain is highly sensitive to cadmium, zinc and cobalt, but resistant to nickel. The ΔnczA strain and the double mutant strain showed reduced growth in the presence of all metals tested. Phylogenetic analysis of the C. crescentus HME-RND proteins showed that CzrA-like proteins, in contrast to those similar to NczA, are almost exclusively found in the Alphaproteobacteria group, and the characteristic protein signatures of each group were highlighted.

Conclusions

The czrCBA efflux system is involved mainly in response to cadmium and zinc with a secondary role in response to cobalt. The nczCBA efflux system is involved mainly in response to nickel and cobalt, with a secondary role in response to cadmium and zinc. CzrA belongs to the HME2 subfamily, which is almost exclusively found in the Alphaproteobacteria group, as shown by phylogenetic analysis. NczA belongs to the HME1 subfamily which is more widespread among diverse Proteobacteria groups. Each of these subfamilies present distinctive amino acid signatures.

Global gene expression under nitrogen starvation in Xylella fastidiosa: contribution of the σ54 regulon

BACKGROUND

Xylella fastidiosa, a Gram-negative fastidious bacterium, grows in the xylem of several plants causing diseases such as citrus variegated chlorosis. As the xylem sap contains low concentrations of amino acids and other compounds, X. fastidiosa needs to cope with nitrogen limitation in its natural habitat.

RESULTS

In this work, we performed a whole-genome microarray analysis of the X. fastidiosa nitrogen starvation response. A time course experiment (2, 8 and 12 hours) of cultures grown in defined medium under nitrogen starvation revealed many differentially expressed genes, such as those related to transport, nitrogen assimilation, amino acid biosynthesis, transcriptional regulation, and many genes encoding hypothetical proteins. In addition, a decrease in the expression levels of many genes involved in carbon metabolism and energy generation pathways was also observed. Comparison of gene expression profiles between the wild type strain and the rpoN null mutant allowed the identification of genes directly or indirectly induced by nitrogen starvation in a σ54-dependent manner. A more complete picture of the σ54 regulon was achieved by combining the transcriptome data with an in silico search for potential σ54-dependent promoters, using a position weight matrix approach. One of these σ54-predicted binding sites, located upstream of the glnA gene (encoding glutamine synthetase), was validated by primer extension assays, confirming that this gene has a σ54-dependent promoter.

CONCLUSIONS

Together, these results show that nitrogen starvation causes intense changes in the X. fastidiosa transcriptome and some of these differentially expressed genes belong to the σ54 regulon.

Catalase-peroxidase activity is decreased in a Caulobacter crescentus rho mutant

A Caulobacter crescentus rho::Tn5 mutant strain presenting a partially functional transcription termination factor Rho is highly sensitive to hydrogen peroxide in both exponential and stationary phases. The mutant was shown to be permanently under oxidative stress, based on fluorophore oxidation, and also to be sensitive to tert-butyl hydroperoxide and paraquat. However, the results showed that the activities of superoxide dismutases CuZnSOD and FeSOD and the alkylhydroperoxide reductase ahpC mRNA levels in the rho mutant were comparable to the wild-type control in the exponential and stationary phases. In contrast, the KatG catalase activity of the rho mutant strain was drastically decreased and did not show the expected increase in the stationary phase compared with the exponential phase. Transcription of the katG gene was increased in the rho mutant and the levels of the immunoreactive KatG protein do not differ considerably compared with the wild type in the stationary phase, suggesting that KatG activity is affected in a translational or a post-translational step.

Fur controls iron homeostasis and oxidative stress defense in the oligotrophic alpha-proteobacterium Caulobacter crescentus

In most bacteria, the ferric uptake regulator (Fur) is a global regulator that controls iron homeostasis and other cellular processes, such as oxidative stress defense. In this work, we apply a combination of bioinformatics, in vitro and in vivo assays to identify the Caulobacter crescentus Fur regulon. A C. crescentus fur deletion mutant showed a slow growth phenotype, and was hypersensitive to H(2)O(2) and organic peroxide. Using a position weight matrix approach, several predicted Fur-binding sites were detected in the genome of C. crescentus, located in regulatory regions of genes not only involved in iron uptake and usage but also in other functions. Selected Fur-binding sites were validated using electrophoretic mobility shift assay and DNAse I footprinting analysis. Gene expression assays revealed that genes involved in iron uptake were repressed by iron-Fur and induced under conditions of iron limitation, whereas genes encoding iron-using proteins were activated by Fur under conditions of iron sufficiency. Furthermore, several genes that are regulated via small RNAs in other bacteria were found to be directly regulated by Fur in C. crescentus. In conclusion, Fur functions as an activator and as a repressor, integrating iron metabolism and oxidative stress response in C. crescentus.

Characterization of Caulobacter crescentus response to low temperature and identification of genes involved in freezing resistance

Free-living bacteria must respond to a wide range of temperature changes, and have developed specific mechanisms to survive in extreme environments. In this work we describe a remarkable resistance of mesophilic bacterium Caulobacter crescentus to several cycles of freezing at -80 degrees C, which was able to grow at low temperatures. Exponentially growing cells and late stationary-phase cells presented higher freezing resistance at both -20 and -80 degrees C than early stationary-phase cells. Cryotolerance was observed when log-phase cultures grown at 30 degrees C were preincubated at 5, 15 or 20 degrees C before freezing at -20 degrees C. A transposon library was screened to identify mutants sensitive to freezing at -80 degrees C and three strains presenting <10% survival were isolated. Identification of genes disrupted in each mutant showed that they encoded an AddA family DNA helicase, a DEAD/DEAH box RNA helicase and a putative RND (resistance, nodulation, cell division) efflux system component. These strains showed longer generation times than wild-type cells when growing at 15 degrees C, with the RNA helicase mutant presenting a severe growth defect. These analyses suggest that the singular intrinsic resistance to freezing of C. crescentus is in fact a consequence of several independent traits, especially the maintenance of a proper degree of supercoiling of nucleic acids.

Role of sigma54 in the regulation of genes involved in type I and type IV pili biogenesis in Xylella fastidiosa

The phytopathogen Xylella fastidiosa produces long type IV pili and short type I pili involved in motility and adhesion. In this work, we have investigated the role of sigma factor sigma(54) (RpoN) in the regulation of fimbrial biogenesis in X. fastidiosa. An rpoN null mutant was constructed from the non-pathogenic citrus strain J1a12, and microarray analyses of global gene expression comparing the wild type and rpoN mutant strains showed few genes exhibiting differential expression. In particular, gene pilA1 (XF2542), which encodes the structural pilin protein of type IV pili, showed decreased expression in the rpoN mutant, whereas two-fold higher expression of an operon encoding proteins of type I pili was detected, as confirmed by quantitative RT-PCR (qRT-PCR) analysis. The transcriptional start site of pilA1 was determined by primer extension, downstream of a sigma(54)-dependent promoter. Microarray and qRT-PCR data demonstrated that expression of only one of the five pilA paralogues, pilA1, was significantly reduced in the rpoN mutant. The rpoN mutant made more biofilm than the wild type strain and presented a cell-cell aggregative phenotype. These results indicate that sigma(54) differentially regulates genes involved in type IV and type I fimbrial biogenesis, and is involved in biofilm formation in X. fastidiosa.

The single extracytoplasmic-function sigma factor of Xylella fastidiosa is involved in the heat shock response and presents an unusual regulatory mechanism

Genome sequence analysis of the bacterium Xylella fastidiosa revealed the presence of two genes, named rpoE and rseA, predicted to encode an extracytoplasmic function (ECF) sigma factor and an anti-sigma factor, respectively. In this work, an rpoE null mutant was constructed in the citrus strain J1a12 and shown to be sensitive to exposure to heat shock and ethanol. To identify the X. fastidiosa sigma(E) regulon, global gene expression profiles were obtained by DNA microarray analysis of bacterial cells under heat shock, identifying 21 sigma(E)-dependent genes. These genes encode proteins belonging to different functional categories, such as enzymes involved in protein folding and degradation, signal transduction, and DNA restriction modification and hypothetical proteins. Several putative sigma(E)-dependent promoters were mapped by primer extension, and alignment of the mapped promoters revealed a consensus sequence similar to those of ECF sigma factor promoters of other bacteria. Like other ECF sigma factors, rpoE and rseA were shown to comprise an operon in X. fastidiosa, together with a third open reading frame (XF2241). However, upon heat shock, rpoE expression was not induced, while rseA and XF2241 were highly induced at a newly identified sigma(E)-dependent promoter internal to the operon. Therefore, unlike many other ECF sigma factors, rpoE is not autoregulated but instead positively regulates the gene encoding its putative anti-sigma factor.

Effects of subinhibitory concentrations of clindamycin on the morphological, biochemical and genetic characteristics ofBacteroides fragilis

The effects of subinhibitory concentrations of clindamycin on the morphological, biochemical and genetic characteristics of species of the Bacteroides fragilis group isolated from children with diarrhea were determined. The minimal inhibitory and subinhibitory concentrations for clindamycin were determined. Minimal inhibitory concentration values ranging from 0.25 to 512 microg mL(-1) were observed. Cultures grown with clindamycin were used to determine the macroscopic morphological characteristics, cellular viability, ultrastructural characteristics and DNA integrity. Clindamycin did not alter colonial morphology, but after 6 h elongated cells were observed. Also, extracellular vesicles and electron-lucent areas inside the cytoplasm were observed. Bacteria treated with clindamycin also showed fragmentation of DNA as determined by electrophoresis. The alterations produced by clindamycin might be indicative of a possible modification of the structures involved in bacterial pathogenesis.

Genes involved in cadmium resistance inCaulobacter crescentus

A transposon library of 5700 mutants was constructed in Caulobacter crescentus strain NA1000. Thirteen mutant strains were isolated that presented growth deficiency in the presence of 20 microM cadmium chloride, and the disrupted genes were identified by DNA sequencing. Among the predicted products were found two putative transcriptional regulators, one histidine kinase/response regulator hybrid protein, two 2Fe-2S family proteins, and also members of a putative metal efflux system. Certain of these mutants also showed an increased sensitivity to zinc and copper, and the genes encoding the efflux system were induced by cadmium and zinc, indicating that this system may be important for homeostasis of both metal ions.

The transcription termination factor Rho is essential and autoregulated in Caulobacter crescentus

The impossibility of obtaining a rho null mutant and sensitivity to bicyclomycin have indicated that rho is essential for the viability of Caulobacter crescentus. Transcription gene fusions of sequences with serial deletions of the rho 5' untranslated region (5'-UTR) with a lacZ reporter gene indicated that rho is autoregulated at the level of attenuation of transcription in the 5'-UTR.

An SOS-regulated operon involved in damage-inducible mutagenesis in Caulobacter crescentus

DNA polymerases of the Y-family, such as Escherichia coli UmuC and DinB, are specialized enzymes induced by the SOS response, which bypass lesions allowing the continuation of DNA replication. umuDC orthologs are absent in Caulobacter crescentus and other bacteria, raising the question about the existence of SOS mutagenesis in these organisms. Here, we report that the C.crescentus dinB ortholog is not involved in damage-induced mutagenesis. However, an operon composed of two hypothetical genes and dnaE2, encoding a second copy of the catalytic subunit of Pol III, is damage inducible in a recA-dependent manner, and is responsible for most ultraviolet (UV) and mitomycin C-induced mutations in C.crescentus. The results demonstrate that the three genes are required for the error-prone processing of DNA lesions. The two hypothetical genes were named imuA and imuB, after inducible mutagenesis. ImuB is similar to proteins of the Y-family of polymerases, and possibly cooperates with DnaE2 in lesion bypass. The mutations arising as a consequence of the activity of the imuAB dnaE2 operon are rather unusual for UV irradiation, including G:C to C:G transversions.

Identification and transcriptional control of Caulobacter crescentus genes encoding proteins containing a cold shock domain

The cold shock proteins are small peptides that share a conserved domain, called the cold shock domain (CSD), that is important for nucleic acid binding. The Caulobacter crescentus genome has four csp genes that encode proteins containing CSDs. Three of these (cspA, cspB, and cspC) encode peptides of about 7 kDa and are very similar to the cold shock proteins of other bacteria. Analysis by reverse transcription-PCR of the fourth gene (cspD), which was previously annotated as encoding a 7-kDa protein, revealed that the mRNA is larger and probably encodes a putative 21-kDa protein, containing two CSDs. A search in protein sequences databases revealed that this new domain arrangement has thus far only been found among deduced peptides of alpha-proteobacteria. Expression of each Caulobacter csp gene was studied both in response to cold shock and to growth phase, and we have found that only cspA and cspB are induced by cold shock, whereas cspC and cspD are induced at stationary phase, with different induction rates. The transcription start sites were determined for each gene, and a deletion mapping of the cspD promoter region defined a sequence required for maximal levels of expression, indicating that regulation of this gene occurs at the transcriptional level. Deletion of cspA, but not cspD, caused a reduction in viability when cells were incubated at 10 degrees C for prolonged times, suggesting that cspA is important for adaptation to a low temperature.

Comparative genomics of two Leptospira interrogans serovars reveals novel insights into physiology and pathogenesis

Leptospira species colonize a significant proportion of rodent populations worldwide and produce life-threatening infections in accidental hosts, including humans. Complete genome sequencing of Leptospira interrogans serovar Copenhageni and comparative analysis with the available Leptospira interrogans serovar Lai genome reveal that despite overall genetic similarity there are significant structural differences, including a large chromosomal inversion and extensive variation in the number and distribution of insertion sequence elements. Genome sequence analysis elucidates many of the novel aspects of leptospiral physiology relating to energy metabolism, oxygen tolerance, two-component signal transduction systems, and mechanisms of pathogenesis. A broad array of transcriptional regulation proteins and two new families of afimbrial adhesins which contribute to host tissue colonization in the early steps of infection were identified. Differences in genes involved in the biosynthesis of lipopolysaccharide O side chains between the Copenhageni and Lai serovars were identified, offering an important starting point for the elucidation of the organism's complex polysaccharide surface antigens. Differences in adhesins and in lipopolysaccharide might be associated with the adaptation of serovars Copenhageni and Lai to different animal hosts. Hundreds of genes encoding surface-exposed lipoproteins and transmembrane outer membrane proteins were identified as candidates for development of vaccines for the prevention of leptospirosis.

Insertional Transposon Mutagenesis in the Xylella fastidiosa Citrus Variegated Chlorosis Strain with Transposome

To overcome the difficulty in obtaining mutants of the citrus strains of Xylella fastidiosa, we evaluated mutagenesis using the transposome system as a tool for the isolation of a large number of mutants. Electroporation of a commercial transposome system in X. fastidiosa CVC (Citrus Variegated Chlorosis) strain J1a12 yielded an efficiency of 1.2 x 10(3) kanamycin (Km)-resistant clones per microg of DNA. Southern blot analysis demonstrated that the transposon was randomly inserted, and nucleotide sequence analysis indicated the presence of 9 bp direct repeats flanking the transposon insertion site. Analysis by PCR of one of the insertion mutants (clone J15) showed that the transposon was stable after eight passages in solid media. These results show that the transposome system can be used to generate a random mutant library of Xylella fastidiosa CVC strain.

Genome features of Leptospira interrogans serovar Copenhageni

We report novel features of the genome sequence of Leptospira interrogans serovar Copenhageni, a highly invasive spirochete. Leptospira species colonize a significant proportion of rodent populations worldwide and produce life-threatening infections in mammals. Genomic sequence analysis reveals the presence of a competent transport system with 13 families of genes encoding for major transporters including a three-member component efflux system compatible with the long-term survival of this organism. The leptospiral genome contains a broad array of genes encoding regulatory system, signal transduction and methyl-accepting chemotaxis proteins, reflecting the organism's ability to respond to diverse environmental stimuli. The identification of a complete set of genes encoding the enzymes for the cobalamin biosynthetic pathway and the novel coding genes related to lipopolysaccharide biosynthesis should bring new light to the study of Leptospira physiology. Genes related to toxins, lipoproteins and several surface-exposed proteins may facilitate a better understanding of the Leptospira pathogenesis and may serve as potential candidates for vaccine.

Isolation and characterization of NaCl-sensitive mutants of Caulobacter crescentus

An attempt to characterize Caulobacter crescentus genes important for the response to high concentrations of NaCl was initiated by the isolation of mutants defective in survival in the presence of 85 mM NaCl. A transposon Tn5 library was screened, and five strains which contained different genes disrupted by the transposon were isolated. Three of the mutants had the Tn5 in genes involved in lipopolysaccharide biosynthesis, one had the Tn5 in the nhaA gene, which encodes a Na(+)/H(+) antiporter, and one had the Tn5 in the ppiD gene, which encodes a peptidyl-prolyl cis-trans isomerase. All the mutant strains showed severe growth arrest in the presence of 85 mM NaCl, but only the nhaA mutant showed decreased viability under these conditions. All the mutants except the nhaA mutant showed a slightly reduced viability in the presence of 40 mM KCl, but all the strains showed a more severe reduction in viability in the presence of 150 mM sucrose, suggesting that they are defective in responding to osmotic shock. The promoter regions of each disrupted gene were cloned in lacZ reporter vectors, and the pattern of expression in response to NaCl and sucrose was determined; this showed that both agents induced ppiD and nhaA gene expression but did not induce the other genes. Furthermore, the ppiD gene was not induced by heat shock, indicating that it does not belong to the sigma(32) regulon, as opposed to what was observed for its Escherichia coli homolog.

Site-directed gene disruption in Xylella fastidiosa

As a first approach to generate mutations by DNA insertion, we have developed a shuttle vector, called pSP3, which replicates both in Escherichia coli and in Xylella. Vector pSP3 was constructed by ligating to the E. coli plasmid pBluescript, a kanamycin resistance gene under the control of the Xylella 16S rRNA promoter and the indigenous Xylella plasmid pXF1.3. Transformation experiments have shown that pSP3 replicates stably in Xylella. When a DNA fragment encompassing part of the Xylella xpsD gene was cloned into pSP3, specific integration of the construct into this gene was observed in 10% of the transformants, as early as after two passages of the culture. These results indicate that this vector can be used to generate site-specific gene disruption by homologous recombination in Xylella fastidiosa.

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.

Genetic organization of plasmid pXF51 from the plant pathogen Xylella fastidiosa

The sequence of plasmid pXF51 from the plant pathogen Xylella fastidiosa, the causal agent of citrus variegated chlorosis, has been analyzed. This plasmid codes for 65 open reading frames (ORFs), organized into four main regions, containing genes related to replication, mobilization, and conjugative transfer. Twenty-five ORFs have no counterparts in the public sequence databases, and 7 are similar to conserved hypothetical proteins from other bacteria. A pXF51 incompatibility group has not been determined, as we could not find a typical replication origin. One cluster of conjugation-related genes (trb) seems to be incomplete in pXF51, and a copy of this sequence is found in the chromosome, suggesting it was generated by a duplication event. A second cluster (tra) contains all genes necessary for conjugation transfer to occur, showing a conserved organization with other conjugative plasmids. An identifiable origin of transfer similar to oriT from IncP plasmids is found adjacent to genes encoding two mobilization proteins. None of the ORFs with putative assigned function could be predicted as having a role in pathogenesis, except for a virulence-associated protein D homolog. These results indicate that even though pXF51 appears not to have a direct role in Xylella pathogenesis, it is a conjugative plasmid that could be important for lateral gene transfer in this bacterium. This property may be of great importance for future development of transformation techniques in X. fastidiosa.

The genome sequence of the plant pathogen Xylella fastidiosa. The Xylella fastidiosa Consortium of the Organization for Nucleotide Sequencing and Analysis

Xylella fastidiosa is a fastidious, xylem-limited bacterium that causes a range of economically important plant diseases. Here we report the complete genome sequence of X. fastidiosa clone 9a5c, which causes citrus variegated chlorosis--a serious disease of orange trees. The genome comprises a 52.7% GC-rich 2,679,305-base-pair (bp) circular chromosome and two plasmids of 51,158 bp and 1,285 bp. We can assign putative functions to 47% of the 2,904 predicted coding regions. Efficient metabolic functions are predicted, with sugars as the principal energy and carbon source, supporting existence in the nutrient-poor xylem sap. The mechanisms associated with pathogenicity and virulence involve toxins, antibiotics and ion sequestration systems, as well as bacterium-bacterium and bacterium-host interactions mediated by a range of proteins. Orthologues of some of these proteins have only been identified in animal and human pathogens; their presence in X. fastidiosa indicates that the molecular basis for bacterial pathogenicity is both conserved and independent of host. At least 83 genes are bacteriophage-derived and include virulence-associated genes from other bacteria, providing direct evidence of phage-mediated horizontal gene transfer.

A gene coding for a putative sigma 54 activator is developmentally regulated in Caulobacter crescentus

In Caulobacter crescentus, the alternative sigma factor sigma54 plays an important role in the expression of late flagellar genes. Sigma54-dependent genes are temporally and spatially controlled, being expressed only in the swarmer pole of the predivisional cell. The only sigma54 activator described so far is the FlbD protein, which is involved in activation of the class III and IV flagellar genes and repression of the fliF promoter. To identify new roles for sigma54 in the metabolism and differentiation of C. crescentus, we cloned and characterized a gene encoding a putative sigma54 activator, named tacA. The deduced amino acid sequence from tacA has high similarity to the proteins from the NtrC family of transcriptional activators, including the aspartate residues that are phosphorylated by histidine kinases in other activators. The promoter region of the tacA gene contains a conserved sequence element present in the promoters of class II flagellar genes, and tacA shows a temporal pattern of expression similar to the patterns of these genes. We constructed an insertional mutant that is disrupted in tacA (strain SP2016), and an analysis of this strain showed that it has all polar structures, such as pili, stalk, and flagellum, and displays a motile phenotype, indicating that tacA is not involved in the flagellar biogenesis pathway. However, this strain has a high percentage of filamentous cells and shows a clear-plaque phenotype when infected with phage phiCb5. These results suggest that the TacA protein could mediate the effect of sigma54 on a different pathway in C. crescentus.

Protein factors in Blastocladiella emersonii cell extracts recognize similar sequence elements in the promoters of the genes encoding cAMP-dependent protein kinase subunits

Blastocladiella emersonii contains a single cAMP-dependent protein kinase (PKA), which is similar to the mammalian type II isoforms. Its activity is regulated during development by changes in the levels of the catalytic (C) and regulatory (R) subunits, which occur in parallel with changes in levels of the corresponding mRNAs, suggesting coordinate transcriptional control of the expression of both subunits. Both R and C mRNA levels are low in vegetative cells, rise sharply during sporulation and decrease to basal levels again after germination. To investigate sequence elements common to both Blastocladiella R and C gene promoters, which might be involved in the coordinate regulation of these genes, their 5'-flanking regions were analyzed by gel mobility shift and DNase I footprinting assays. We determined that different DNA-protein complexes are generated when fragments of the R and C gene promoters are incubated with extracts from cells expressing (sporulating cells) or not expressing (vegetative cells) both subunits, and competition experiments suggested that similar protein factors bind to both promoters. DNase I footprinting experiments have indicated that a sequence common to both R and C promoters, and similar to mammalian E-boxes, binds factors present in extracts from vegetative and sporulating cells, whereas sequences flanking the E-boxes in both promoters showed a change in the pattern of DNase I digestion only when the vegetative cell extract was used. This result suggests that the composition of the protein complexes binding to these regions changes during sporulation.

Activation of a temporally regulated Caulobacter promoter by upstream and downstream sequence elements

The flagellar genes of Caulobacter crescentus are expressed under cell-cycle control. Expression is regulated by both flagellar assembly cues and cell-cycle events. In this paper we define the sequences required for the expression of the flgF operon, a new class of sigma 54 flagellar promoter. This promoter type is expressed in the middle portion of the cell cycle and regulates the expression of basal-body genes. DNase I footprinting and mutagenesis demonstrates that an integration host factor (IHF)-binding site is required for maximal levels of transcription of the flgF promoter. In addition to containing a conventional upstream enhancer element (RE-1), this promoter is unusual in that it also requires sequences (element RE-2) immediately downstream of the transcriptional start site for maximal levels of gene expression. Cell-cycle experiments indicate that RE-1 and RE-2 contribute equally to the regulation of temporal transcription. The presence of two intact elements in the promoter results in a fourfold increase in promoter activity compared with a promoter containing only one intact element, suggesting that these two elements may function synergistically to activate transcription.

Regulation of cellular differentiation in Caulobacter crescentus

In Caulobacter crescentus, asymmetry is generated in the predivisional cell, resulting in the formation of two distinct cell types upon cell division: a motile swarmer cell and a sessile stalked cell. These progeny cell types differ in their relative programs of gene expression and DNA replication. In progeny swarmer cells, DNA replication is silenced for a defined period, but stalked cells reinitiate chromosomal DNA replication immediately following cell division. The establishment of these differential programs of DNA replication may be due to the polar localization of DNA replication proteins, differences in chromosome higher-order structure, or pole-specific transcription. The best-understood aspect of Caulobacter development is biogenesis of the polar flagellum. The genes encoding the flagellum are expressed under cell cycle control predominantly in the predivisional cell type. Transcription of flagellar genes is regulated by a trans-acting hierarchy that responds to both flagellar assembly and cell cycle cues. As the flagellar genes are expressed, their products are targeted to the swarmer pole of the predivisional cell, where assembly occurs. Specific protein targeting and compartmentalized transcription are two mechanisms that contribute to the positioning of flagellar gene products at the swarmer pole of the predivisional cell.