Regulation of the "tetCD" genes of transposon Tn10

The Mar, Sox, and Rob Systems

Environments inhabited by Enterobacteriaceae are diverse and often stressful. This is particularly true for Escherichia coli and Salmonella during host association in the gastrointestinal systems of animals. There, E. coli and Salmonella must survive exposure to various antimicrobial compounds produced or ingested by their host. A myriad of changes to cellular physiology and metabolism are required to achieve this feat. A central regulatory network responsible for sensing and responding to intracellular chemical stressors like antibiotics are the Mar, Sox, and Rob systems found throughout the Enterobacteriaceae. Each of these distinct regulatory networks controls expression of an overlapping set of downstream genes whose collective effects result in increased resistance to a wide array of antimicrobial compounds. This collection of genes is known as the mar-sox-rob regulon. This review will provide an overview of the mar-sox-rob regulon and molecular architecture of the Mar, Sox, and Rob systems.

Regulation of transcription from two ssrS promoters in 6S RNA biogenesis

ssrS-encoded 6S RNA is an abundant noncoding RNA that binds σ(70)-RNA polymerase and regulates expression at a subset of promoters in Escherichia coli. It is transcribed from two tandem promoters, ssrS P1 and ssrS P2. Regulation of transcription from two ssrS promoters in 6S RNA biogenesis was examined. Both P1 and P2 were growth phase-dependently regulated. Depletion of 6S RNA had no effect on growth-phase-dependent transcription from either promoter, whereas overexpression of 6S RNA increased P1 transcription and decreased P2 transcription, suggesting that transcription from P1 and P2 is subject to feedback activation and feedback inhibition, respectively. This feedback regulation disappeared in Δfis strains, supporting involvement of Fis in this process. The differential feedback regulation may provide a means for maintaining appropriate cellular concentrations of 6S RNA.

A novel fluorescent reporter system for monitoring and identifying RNase III activity and its target RNAs

Bacteriophage vectors for achieving single-copy gene expression linked to a colorigenic reporter assay have been used successfully for genetic screening applications. However, the limited number of cloning sites in these vectors, combined with the requirement for lac- strains and the time- and/or media-dependence of the chemical-based colorimetric reaction, have limited the range of applications for these vectors. An alternative approach using a fluorescent reporter gene such as green fluorescent protein (GFP) or GFP derivatives could overcome some of these technical issues and facilitate real-time monitoring of promoter and/or protein activity. Here, we report the development of a novel translational bacteriophage fusion vector encoding enhanced GFP (eGFP) that can be incorporated into the chromosome as a single-copy gene. We identified a Bacillus promoter (BP) that is stably expressed in Escherichia coli and drives ~6-fold more expression of eGFP than the T7 promoter in the absence of inducer. Incorporating this BP and RNase III target signals into a single system enabled clear detection of the absence or downregulation of RNase III activity in vivo, thereby establishing a system for screening and identifying novel RNase III targets in a matter of days. An RNase III target signal identified in this manner was confirmed by post-transcriptional analysis. We anticipate that this novel translational fusion vector will be used extensively to study activity of both interesting RNases and related complex or to identify or validate targets of RNases that are otherwise difficult to study due to their sensitivity to environmental stresses and/or autoregulatory processes.

Novel conjugative transferable multiple drug resistance plasmid pAQU1 from Photobacterium damselae subsp. damselae isolated from marine aquaculture environment

The emergence of drug-resistant bacteria is a severe problem in aquaculture. The ability of drug resistance genes to transfer from a bacterial cell to another is thought to be responsible for the wide dissemination of these genes in the aquaculture environment; however, little is known about the gene transfer mechanisms in marine bacteria. In this study, we show that a tetracycline-resistant strain of Photobacterium damselae subsp. damselae, isolated from seawater at a coastal aquaculture site in Japan, harbors a novel multiple drug resistance plasmid. This plasmid named pAQU1 can be transferred to Escherichia coli by conjugation. Nucleotide sequencing showed that the plasmid was 204,052 base pairs and contained 235 predicted coding sequences. Annotation showed that pAQU1 did not have known repA, suggesting a new replicon, and contained seven drug resistance genes: bla(CARB-9)-like, floR, mph(A)-like, mef(A)-like, sul2, tet(M) and tet(B). The plasmid has a complete set of genes encoding the apparatus for the type IV secretion system with a unique duplication of traA. Phylogenetic analysis of the deduced amino acid sequence of relaxase encoded by traI in pAQU1 demonstrated that the conjugative transfer system of the plasmid belongs to MOB(H12), a sub-group of the MOB(H) plasmid family, closely related to the IncA/C type of plasmids and SXT/R391 widely distributed among species of Enterobacteriaceae and Vibrionaceae. Our data suggest that conjugative transfer is involved in horizontal gene transfer among marine bacteria and provide useful insights into the molecular basis for the dissemination of drug resistance genes among bacteria in the aquaculture environment.

An excretory function for the Escherichia coli outer membrane pore TolC: upregulation of marA and soxS transcription and Rob activity due to metabolites accumulated in tolC mutants

Efflux pumps function to rid bacteria of xenobiotics, including antibiotics, bile salts, and organic solvents. TolC, which forms an outer membrane channel, is an essential component of several efflux pumps in Escherichia coli. We asked whether TolC has a role during growth in the absence of xenobiotics. Because tolC transcription is activated by three paralogous activators, MarA, SoxS, and Rob, we examined the regulation of these activators in tolC mutants. Using transcriptional fusions, we detected significant upregulation of marRAB and soxS transcription and Rob protein activity in tolC mutants. Three mechanisms could be distinguished: (i) activation of marRAB transcription was independent of marRAB, soxR, and rob functions; (ii) activation of soxS transcription required SoxR, a sensor of oxidants; and (iii) Rob protein was activated posttranscriptionally. This mechanism is similar to the mechanisms of upregulation of marRAB, soxS, and Rob by treatment with certain phenolics, superoxides, and bile salts, respectively. The transcription of other marA/soxS/rob regulon promoters, including tolC itself, was also elevated in tolC mutants. We propose that TolC is involved in the efflux of certain cellular metabolites, not only xenobiotics. As these metabolites accumulate during growth, they trigger the upregulation of MarA, SoxS, and Rob, which in turn upregulate tolC and help rid the bacteria of these metabolites, thereby restoring homeostasis.

Two functions of the C-terminal domain of Escherichia coli Rob: mediating "sequestration-dispersal" as a novel off-on switch for regulating Rob's activity as a transcription activator and preventing degradation of Rob by Lon protease

In Escherichia coli, Rob activates transcription of the SoxRS/MarA/Rob regulon. Previous work revealed that Rob resides in three to four immunostainable foci, that dipyridyl and bile salts are inducers of its activity, and that inducers bind to Rob's C-terminal domain (CTD). We propose that sequestration inactivates Rob by blocking its access to the transcriptional machinery and that inducers activate Rob by mediating its dispersal, allowing interaction with RNA polymerase. To test "sequestration-dispersal" as a new mechanism for regulating the activity of transcriptional activators, we fused Rob's CTD to SoxS and used indirect immunofluorescence microscopy to determine the effect of inducers on SoxS-Rob's cellular localization. Unlike native SoxS, which is uniformly distributed throughout the cell, SoxS-Rob is sequestered without an inducer, but is rapidly dispersed when cells are treated with an inducer. In this manner, Rob's CTD serves as an anti-sigma factor in regulating the co-sigma-factor-like activity of SoxS when fused to it. Rob's CTD also protects its N-terminus from Lon protease, since Lon's normally rapid degradation of SoxS is blocked in the chimera. Accordingly, Rob's CTD has novel regulatory properties that can be bestowed on another E. coli protein.

Concert of regulators to switch on LEE expression in enterohemorrhagic Escherichia coli O157:H7: Interplay between Ler, GrlA, HNS and RpoS

Enterohemorrhagic (EHEC) and enteropathogenic (EPEC) Escherichia coli strains carry a pathogenicity island termed locus of enterocyte effacement (LEE) responsible for attaching and effacing lesions on epithelial cells. The expression of LEE varies among isolates and is dependent on environmental cues. In the EHEC O157:H7 Sakaï isolate (RIMD-0509952 strain), we found that the non-coding RNA, DsrA, activates the expression of the LEE. This activation requires RpoS, the stress sigma factor. The DsrA/RpoS regulatory pathway mediates its positive effect by stimulating the transcription of ler, a positive regulatory gene encoded by the LEE. A second regulatory pathway, repressed by HNS, is also able to activate the transcription of ler and requires GrlA, another LEE-encoded regulator. Both regulatory pathways, DsrA/RpoS and HNS/GrlA, affect the activity of the ler distal promoter and require the Ler protein to be functional. Our data demonstrate that the LEE expression can be turned on by at least two separate pathways acting on the transcription of ler.

Characterization of TetD as a transcriptional activator of a subset of genes of the Escherichia coli SoxS/MarA/Rob regulon

In Escherichia coli, SoxS, MarA and Rob form a closely related subset of the AraC/XylS family of positive regulators, sharing approximately 42% amino acid sequence identity over the length of SoxS and the ability to activate transcription of a common set of target genes that provide resistance to redox-cycling compounds and antibiotics. On the basis of its approximately 43% amino acid sequence identity with SoxS, MarA and Rob, TetD, encoded by transposon Tn10, appears to be a fourth member of the subset. However, although its expression has been shown to be negatively regulated by TetC and not inducible by tetracycline, the physiological function of TetD is unknown. Accordingly, in the work presented here, we initiate a molecular characterization of TetD. We show that expression of TetD activates transcription of a subset of the SoxS/MarA/Rob regulon genes and confers resistance to redox-cycling compounds and antibiotics. We show that mutations in the putative TetD binding site of a TetD-activatable promoter and a mutation in the protein's N-terminal DNA recognition helix interfere with transcription activation, thereby indicating that TetD directly activates target gene transcription. Finally, we show that TetD, like SoxS and MarA, is intrinsically unstable; however, unlike SoxS and MarA, TetD is not degraded by Lon or any of the cell's known cytoplasmic ATP-dependent proteases. Thus, we conclude that TetD is a bona fide member of the SoxS/MarA/Rob subfamily of positive regulators.

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.

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.

Cold-temperature induction of Escherichia coli polynucleotide phosphorylase occurs by reversal of its autoregulation

When Escherichia coli cells are shifted to low temperatures (e.g. 15 degrees C), growth halts while the 'cold shock response' (CSR) genes are induced, after which growth resumes. One CSR gene, pnp, encodes polynucleotide phosphorylase (PNPase), a 3'-exoribonuclease and component of the RNA degradosome. At 37 degrees C, ribonuclease III (RNase III, encoded by rnc) cleaves the pnp untranslated leader, whereupon PNPase represses its own translation by an unknown mechanism. Here, we show that PNPase cold-temperature induction involves several post-transcriptional events, all of which require the intact pnp mRNA leader. The bulk of induction results from reversal of autoregulation at a step subsequent to RNase III cleavage of the pnp leader. We also found that pnp translation occurs throughout cold-temperature adaptation, whereas lacZ(+) translation was delayed. This difference is striking, as both mRNAs are greatly stabilized upon the shift to 15 degrees C. However, unlike the lacZ(+) mRNA, which remains stable during adaptation, pnp mRNA decay accelerates. Together with other evidence, these results suggest that mRNA is generally stabilized upon a shift to cold temperatures, but that a CSR mRNA-specific decay process is initiated during adaptation.

Analysis of the complete nucleotide sequence of the tetracycline-resistance transposon Tn10

An analysis of the complete nucleotide sequence of the composite tetracycline-resistance transposon Tn10 (9147 bp) from the Salmonella typhi conjugative plasmid R27 is presented. A comparison of the protein sequences from IS10-right and IS10-left transposases has identified four amino acid differences. These residues appear to play an important role in normal transposase function and may account for the differences in exhibited transposition activities. The tetracycline determinants encoded by this version of Tn10 share >99% identity with those of Tn10(R100), demonstrating the conservation that exists between these transposons. A previously uncharacterized approximately 3000-bp region of Tn10 contains four putative open reading frames. One of these open reading frames shares 55% identity with the glutamate permease protein sequence from Haemophilus influenzae although it was unable to complement an Escherichia coli glutamate permease mutant, with which it shares 51% identity. The three remaining putative open reading frames are arranged as a discrete genetic unit adjacent to the glutamate permease homolog and are transcribed in the opposite direction. Two of these open reading frames are homologous with Bacillus subtilis proteins of unknown functions while the other has no homologs in the database. The presence of an aminoacyl-tRNA synthetase class II motif in one of these open reading frames in combination with the glutamate permease homolog allows us to postulate that this region of Tn10 could once have played a role in amino acid metabolism.