Adaptation of the gut pathobiont Enterococcus faecalis to deoxycholate and taurocholate bile acids.

Enterococcus faecalis is a natural inhabitant of the human gastrointestinal tract. This bacterial species is subdominant in a healthy physiological state of the gut microbiota (eubiosis) in adults, but can become dominant and cause infections when the intestinal homeostasis is disrupted (dysbiosis). The relatively high concentrations of bile acids deoxycholate (DCA) and taurocholate (TCA) hallmark eubiosis and dysbiosis, respectively. This study aimed to better understand how E. faecalis adapts to DCA and TCA. We showed that DCA impairs E. faecalis growth and possibly imposes a continuous adjustment in the expression of many essential genes, including a majority of ribosomal proteins. This may account for slow growth and low levels of E. faecalis in the gut. In contrast, TCA had no detectable growth effect. The evolving transcriptome upon TCA adaptation showed the early activation of an oligopeptide permease system (opp2) followed by the adjustment of amino acid and nucleotide metabolisms. We provide evidence that TCA favors the exploitation of oligopeptide resources to fuel amino acid needs in limiting oligopeptide conditions. Altogether, our data suggest that the combined effects of decreased DCA and increased TCA concentrations can contribute to the rise of E. faecalis population during dysbiosis.

Temperature sensing by the dsrA promoter.

Synthesis of the small regulatory RNA DsrA is under temperature control. The minimal dsrA promoter of 36 bp contains sufficient information to ensure such regulation. In vivo, we have analyzed the critical elements responsible for the temperature control of dsrA by using a collection of chimeric promoters combining various elements of the dsrA promoter and the lacUV5 promoter, which does not respond to temperature. Our results favor an RNA polymerase-DNA interaction model instead of a trans-acting factor for temperature regulation. While all of the elements of the dsrA promoter contribute to temperature-sensitive expression, the sequence of the -10 box and the spacer region are the essential elements for the thermal response of the dsrA promoter. The proper context for these promoter elements, including at least one of the flanking elements, the -35 region or the start site region, is also required. Point mutations demonstrate that the sequence of the -10 box imposes constraints on the length and the sequence of the spacer and/or its AT richness, even at low temperature. These results show a complex interdependence of different regions in the promoter for temperature regulation.

Small non-coding RNAs, co-ordinators of adaptation processes in Escherichia coli: the RpoS paradigm.

Adaptation to the changing environment requires both the integration of external signals and the co-ordination of internal responses. Around 50 non-coding small RNAs (sRNAs) have been described in Escherichia coli; the levels of many of these vary with changing environmental conditions. This suggests that they play a role in cell adaptation. In this review, we use the regulation of RpoS (sigma38) translation as a paradigm of sRNA-mediated response to environmental conditions; rpoS is currently the only known gene regulated post-transcriptionally by at least three sRNAs. DsrA and RprA stimulate RpoS translation in response to low temperature and cell surface stress, respectively, whereas OxyS represses RpoS translation in response to oxidative shock. However, in addition to regulating RpoS translation, DsrA represses the translation of HNS (a global regulator of gene expression), whereas OxyS represses the translation of FhlA (a transcriptional activator), allowing the cell to co-ordinate different pathways involved in cell adaptation. Environmental cues affect the synthesis and stability of specific sRNAs, resulting in specific sRNA-dependent translational control.

Identification of novel small RNAs using comparative genomics and microarrays.

A burgeoning list of small RNAs with a variety of regulatory functions has been identified in both prokaryotic and eukaryotic cells. However, it remains difficult to identify small RNAs by sequence inspection. We used the high conservation of small RNAs among closely related bacterial species, as well as analysis of transcripts detected by high-density oligonucleotide probe arrays, to predict the presence of novel small RNA genes in the intergenic regions of the Escherichia coli genome. The existence of 23 distinct new RNA species was confirmed by Northern analysis. Of these, six are predicted to encode short ORFs, whereas 17 are likely to be novel functional small RNAs. We discovered that many of these small RNAs interact with the RNA-binding protein Hfq, pointing to a global role of the Hfq protein in facilitating small RNA function. The approaches used here should allow identification of small RNAs in other organisms.

Signal transduction cascade for regulation of RpoS: temperature regulation of DsrA.

Many environmental parameters modulate the amount of the RpoS sigma factor in Escherichia coli. Temperature control of RpoS depends on the untranslated RNA DsrA. DsrA activates RpoS translation by pairing with the leader of the mRNA. We find that temperature affects both the rate of transcription initiation of the dsrA gene and the stability of DsrA RNA. Both are increased at low temperature (25 degrees C) compared to 37 or 42 degrees C. The combination of these results is 25-fold-less DsrA at 37 degrees C and 30-fold less at 42 degrees C than at 25 degrees C. Using an adapted lacZ-based reporter system, we show that temperature control of transcription initiation of dsrA requires only the minimal promoter of 36 bp. Overall, transcription responses to temperature lead to a sixfold increase in DsrA synthesis at 25 degrees C over that at 42 degrees C. Furthermore, two activating regions and a site for LeuO negative regulation were identified in the dsrA promoter. The activating regions also activate transcription in vitro. DsrA decays with a half-life of 23 min at 25 degrees C and 4 min at 37 and 42 degrees C. These results demonstrate that the dsrA promoter and the stability of DsrA RNA are the thermometers for RpoS temperature sensing. Multiple inputs to DsrA accumulation allow sensitive modulation of changes in the synthesis of the downstream targets of DsrA such as RpoS.

Involvement of differential efficiency of transcription by esigmas and esigma70 RNA polymerase holoenzymes in growth phase regulation of the Escherichia coli osmE promoter.

Transcription of the gene osmE of Escherichia coli is inducible by elevated osmotic pressure and during the decelerating phase of growth. osmE expression is directed by a single promoter, osmEp. Decelerating phase induction of osmEp is dependent on the sigmas (RpoS) factor, whereas its osmotic induction is independent of sigmas. Purified Esigmas and Esigma70 were both able to transcribe osmEp in vitro on supercoiled templates. In the presence of rpoD800, a mutation resulting in a thermosensitive sigma70 factor, a shift to non-permissive temperature abolished induction of osmEp after an osmotic shock during exponential phase, but did not affect the decelerating phase induction. Point mutations affecting osmEp activity were isolated. Down-promoter mutations decreased transcription in both the presence and the absence of sigmas, indicating that the two forms of RNA polymerase holoenzyme recognize very similar sequence determinants on the osmE promoter. Three up-promoter mutations brought osmEp closer to the consensus of Esigma70-dependent promoters. The two variant promoters exhibiting the highest efficiency became essentially independent of sigmas in vivo. Our data suggest that Esigmas transcribes wild-type osmEp with a higher efficiency than Esigma70. A model in which an intrinsic differential recognition contributes to growth phase-dependent regulation is proposed. Generalization of this model to other sigmas-dependent promoters is discussed.

The genome of the pseudo T-even bacteriophages, a diverse group that resembles T4.

Polymerase chain reaction analysis of a large collection of bacteriophages with T-even morphology revealed four phages that are distantly related to all the others. The genomes of these pseudo T-even phages hybridized under stringent conditions to only a limited portion of the T4 genome that encodes virus head, head-to-tail joining and contractile tail genes. Except for this region, no extensive hybridization was detected between most pairs of the different pseudo T-even genomes. Sequencing of this conserved region of the pseudo T-even phage RB49 revealed substantial nucleotide sequence divergence from T4 (approximately 30% to 40%), and random genomic sequencing of this phage indicated that more than a third of its sequences had no detectable homology to T4. Among those sequences related to the T-even genes were virion structural components including the constituents of the phage base plate. Only a few sequences had homology to T4 early functions; these included ribonucleotide diphosphatase reductase, DNA ligase and the large subunit of DNA topoisomerase. The genomes of the pseudo T-even phage were digested by restriction enzymes that are unable to digest the T-even DNAs which contain glucosylated hydroxymethyl-cytosine residues. This suggests that only limited nucleotide modifications must be present in the pseudo T-even genomes. Conservation of much of the morphogenetic region of these diverse phage genomes may reflect particularly strong sequence constraints on these gene products. However, other explanations are considered, including the possibility that the various morphogenetic segments were acquired by the pseudo T-even genomes by modular evolution. These results support the notion that phage evolution may proceed within a network of both closely and distantly related genomes.

Bacteriophage T4 host range is expanded by duplications of a small domain of the tail fiber adhesin.

The adsorption specificity of T4 is determined by the tip of the gene 37 tail fibers which bind to receptors on the bacterial surface. T4 infects only Escherichia coli and closely related Shigella species, but rare host range mutants can be isolated that infect Yersinia pseudotuberculosis I, an evolutionally distant bacterium. Some of these mutations result in amino acid residue substitutions in the C-terminal portion of gene 37, but others involve unequal exchanges between a series of sequence motifs (His boxes) in the same region. The duplication or mutational alteration of this segment apparently suffices for phage adsorption to a Yersinia receptor. It is suggested that recombination between the His box sequences can generate diversity in phage host range by shuffling receptor recognition domains.

Genomic polymorphism in the T-even bacteriophages.

We have compared the genomes of 49 bacteriophages related to T4. PCR analysis of six chromosomal regions reveals two types of local sequence variation. In four loci, we found only two alternative configurations in all the genomes that could be analyzed. In contrast, two highly polymorphic loci exhibit variations in the number, the order and the identity of the sequences present. In phage T4, both highly polymorphic loci encode internal proteins (IPs) that are encapsidated in the phage particle and injected with the viral DNA. Among the various T4-related phages, 10 different ORFs have been identified in the IP loci; their amino acid sequences have the characteristics of internal proteins. At the beginning of each of these coding sequences is a highly conserved 11 amino acid leader motif. In addition, both 5' and 3' to most of these ORFs, there is a approximately 70 bp sequence that contains a T4 early promoter sequence with an overlapping inversely repeated sequence. The homologies within these flanking sequences may mediate the recombinational shuffling of the IP sequences within the locus. A role for the new IP-like sequences in determining the phage host range is proposed since such a role has been previously demonstrated for the IP1 gene of T4.

Direct PCR sequencing of the ndd gene of bacteriophage T4: identification of a product involved in bacterial nucleoid disruption.

The rapid disruption of the Escherichia coli nucleoid after T4 infection requires the activity of the phage-encoded ndd gene. We have genetically identified the sequence encoding ndd. Determination of the sequence of a 2.5-kb segment including ndd closed the last significant gap in the sequence of the T4 genome. This analysis was performed on PCR-amplified fragments that were purified by gel-exclusion chromatography and then submitted to linear amplification cycle sequencing. This technology permitted sequence comparison of two ndd mutants (ndd44 and ndd98) with the wild-type gene. The analysis of ndd from six bacteriophages of the T-even family indicated that the protein encoded by this nonessential gene is surprisingly conserved.

Osmotic induction of the periplasmic trehalase in Escherichia coli K12: characterization of the treA gene promoter.

The Escherichia coli treA gene encodes an osmotically inducible periplasmic trehalase. A strain carrying a treA-lacZ transcriptional fusion was constructed. The beta-galactosidase activity produced in this strain growing exponentially in a medium of high osmotic pressure was 10-fold higher than that produced in a medium of low osmotic pressure, demonstrating that treA transcription is osmotically inducible. treA transcriptional induction depends neither on the presence of trehalase itself nor on the synthesis of cytoplasmic trehalose which occurs in response to osmotic stress in wild-type E. coli strains. The treA promoter was identified by S1 nuclease protection. Deletion analysis demonstrated that sequences sufficient for the osmotic induction lie downstream from nucleotide -40 with respect to the transcription start. Transcription initiation at treAp required the presence of a functional sigma 70 subunit of RNA polymerase. treA expression was increased in the presence of a mutation in osmZ, which was previously identified as leading to a partially constitutive expression of the osmotically inducible proU operon.