 | This articleneeds additional citations forverification. Please helpimprove this article byadding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "RpoS" – news ·newspapers ·books ·scholar ·JSTOR(May 2011) (Learn how and when to remove this message) |
The generpoS (RNApolymerase, sigmaS, also called katF) encodes thesigma factorsigma-38 (σ38, or RpoS), a 37.8 kD protein inEscherichia coli.[2] Sigma factors are proteins that regulatetranscription inbacteria. Sigma factors can be activated in response to different environmental conditions.rpoS is transcribed in late exponential phase, and RpoS is the primary regulator of stationary phase genes. RpoS is a central regulator of the general stress response and operates in both a retroactive and a proactive manner: it not only allows the cell to survive environmental challenges, but it also prepares the cell for subsequent stresses (cross-protection).[3] The transcriptional regulatorCsgD is central tobiofilm formation, controlling the expression of thecurli structural and export proteins, and thediguanylate cyclase, adrA, which indirectly activates cellulose production.[4] TherpoS gene most likely originated in thegammaproteobacteria.[3]
Regulatory mechanisms that control RpoS exist at various levels of gene and protein organization:transcription,translation, degradation, and protein activity. These processes occur in response to stresses such as near-UV radiation,acid, temperature orosmotic shock,oxidative stress, and nutrient deprivation. While many key regulatory entities have been identified in these areas, the precise mechanisms by which they signalrpoS transcription, translation, proteolysis or activity remain largely uncharacterized.
Transcription ofrpoS inE. coli is mainly regulated by the chromosomal rpoSp promoter.[5] rpoSp promotes transcription ofrpoS mRNA, and is induced upon entry intostationary phase in cells growing on rich media via an unknown mechanism.[6] Flanking rpoSp are two putativecAMP-CRP (cyclic AMP-cAMPreceptor protein) binding sites that seem to controlrpoS transcription in an antagonistic manner. The position of the first site upstream of the majorrpoS promoter corresponds to a “classical activator” similarly found in thelac promoter thereby suggesting that its effects on transcription are activating (Lange and Hengge-Aronis, 1994); in contrast, the location of the second cAMP-CRP site is indicative of inhibitory action. In exponential phase,crp mutants exhibit high levels ofrpoS expression, suggesting that cAMP-CRP inhibitsrpoS transcription. Upon entry into stationary phase, on the other hand, cAMP-CRP may upregulaterpoS transcription (Hengge-Aronis, 2002). While these observations may explain the seemingly dual nature of the cAMP-CRP binding sites, they require an explanation of phase-dependent selection of cAMP-CRP site activation to fully account for the contradictory data. Additional regulatory controls forrpoS transcription include: BarA, aHistidine sensorkinase which can activate OmpR and thereby promote porin synthesis; levels of small molecules such asppGppp which may hindertranscriptional elongation or stability in response to amino acid limitation, or carbon, nitrogen or phosphorus starvation (Gentryet al., 1993).[citation needed] Despite the numerous controls onrpoS transcription, cellularrpoS mRNA levels remain high during exponential phase and the majority of extracellularstimuli do not significantly affectrpoS transcription.
Most RpoS expression is determined at the translational level.[7]sRNAs (small noncodingRNAs) sense environmental changes and in turn increaserpoS mRNA translation to allow the cell to accordingly adjust to external stress. The promoter of the 85 nucleotide sRNADsrA contains a temperature-sensitive transcription initiation thermocontrol as it is repressed at high (42˚C) temperatures, but induces (perhaps by complementary binding to)rpoS at low (25˚C) temperatures.[8] Another sRNA,RprA, stimulatesrpoS translation in response to cell surface stress signaled via the RcsCsensor kinase.[8] A third type of sRNA, OxyS, is regulated by OxyR, the primary sensor of oxidative shock.[9] The mechanism by which OxyS interferes withrpoS mRNAtranslational efficiency is not known. However, the RNA-binding proteinHfq is implicated in the process.[10] Hfq binds torpoS mRNAin vitro and may thereby modifyrpoS mRNA structure for optimal translation. Hfq activates both DsrA and RprA. In contrast, LeuO inhibitsrpoS translation by repressingdsrA expression and the histone-like protein HN-S (and its paralog StpA) inhibitsrpoS translation via an unknown mechanism. In addition, H-NS, LeuO, Hfq and DsrA form an interconnected regulatory network that ultimately controlsrpoS translation.
RpoS translation was also shown to be controlled in other bacterial species, beside Escherichia coli. E.g., in the opportunistic human pathogenPseudomonas aeruginosa the sRNA ReaL translationally silences rpoS mRNA.[11]
RpoS proteolysis forms another level of the sigma factor’s regulation. Degradation occurs via ClpXP, a barrel-shaped protease composed of two six-subunit rings of the ATP-dependent ClpX chaperone that surround two seven-subunit rings of ClpP (Repoilaet al., 2003). The response regulator RssB has been identified as a σS-specific recognition factor crucial for RpoS degradation. Additional factors known to regulate RpoS proteolysis but via incompletely characterized mechanisms include: RssA which is found on the same operon as RssB; H-NS and DnaK, both of which also regulaterpoS mRNA translation, and LrhA; and acetyl phosphate affects RpoS proteolysis by possibly acting as a phosphoryl donor to RssB.
Consistent with its role as the master controller of the bacterial stress response, RpoS regulates the expression of stress-response genes that fall into various functional categories: stress resistance, cell morphology,metabolism,virulence andlysis.
Many genes under RpoS control confer stress resistance to assaults such asDNA damage, presence ofreactive oxygen species andosmotic shock. The product ofxthA is an exonuclease that participates in DNA repair by recognizing and removing 5’ monophosphates near abasic sites in damaged DNA.[12] Likewise, catalases HPI and HPII, encoded bykatG andkatE convert harmful hydrogen peroxide molecules to water and oxygen.[13] TheotsBA gene producttrehalose functions as anosmoprotectant and is needed for desiccation resistance.[14] Additional RpoS-dependent factors involved in oxidative stress includeglutathione reductase (encoded bygor), andsuperoxide dismutase (encoded bysodC).[15]
It has also been found, using comparative proteomic analysis withB. pseudomallei, that rpoS regulates eight oxidative responsive proteins including ScoA (a SCOT subunit) not previously known for oxidative stress response involvement. The regulatory effect in this case is RpoS down regulation of SCOT expression in response to oxidative stress inB. pseudomallei.[16]
RpoS-dependent genes involved in changes in cell membrane permeability and general cell morphology mostly belong to theosm family of genes.osmB encodes an outer membrane lipoprotein that may play a role in cell aggregation (Junget al., 1990),[16] whereasosmY encodes a periplasmic protein. Additional RpoS-dependent factors that determine the size and shape of the cell include the morphogenebolA and products of theftsQAZ operon that play a role in the timing of cell division.[5] Control of cell shape, cell division and cell-cell interaction are likely to be important in inhibiting cell proliferation and thus allocating resources to cell survival during periods of stress.
Metabolically-optimal survival conditions include RpoS-dependent decreasedKrebs cycle activity and increased glycolytic activity to limit the reactive oxygen species that are byproduced as a result of essential cellular processes.Pyruvate entry into the Krebs cycle is inhibited by the product of the RpoS-dependent genepoxB. An overall slowdown in metabolic activity is consistent with energy conservation and reduced growth during periods of stress.
As a defense mechanism, the host environment is hostile to invading pathogens. Therefore, infection can be a stressful event for pathogenic bacteria and control of virulence genes may be temporally correlated with the timing of infection by pathogens.[17] Discovery of RpoS-dependent virulence genes inSalmonella is consistent with RpoS as a general regulator of the stress response: thespv gene found on a virulence plasmid in this bacterium is controlled by RpoS and is required for growth in deep lymphoid tissue such as the spleen and liver.[18]
RpoS also plays an important role in regulating cell lysis. Along with OmpR, it upregulates theentericidin (ecnAB) locus which encodes a lysis-inducing toxin.[19] In contrast,ssnA is negatively controlled by RpoS but it also promotes lysis. Paradoxically, lysis is seen as a survival process in certain contexts.