Article Figures & Data
Figures
- FIG. 1.
Various levels of σS regulation are differentially affected by various stress conditions. An increase of the cellular σS level can be obtained either by stimulating σS synthesis at the levels of rpoS transcription or rpoS mRNA translation or by inhibiting σS proteolysis (which under nonstress conditions is extraordinarily rapid). The most rapid and strongest reaction can be achieved by a combination of these processes (as observed, e.g., on hyperosmotic or pH shifts). For further details, see the text.
- FIG. 2.
Transcriptional control regions upstream of the rpoS gene. (A) The nlpD-rpoS operon is located at 61.76 min on the E. coli chromosome, where it is trancribed in counterclockwise direction. (B) The operon promoters (nlpDp1 and nlpDp2) contribute to basal expression of rpoS but are not regulated by growth rate or growth phase (115). (C) The major rpoS promoter (rpoSp) is located within the nlpD gene, is flanked by two putative cAMP-CRP binding sites (CRP box I and II), and is subject to stationary-phase induction when cells are grown on rich medium (112). Broken lines in panel A indicate the relative positions of the sequences shown in panels B and C.
- FIG. 3.
The rpoS translational control network. rpoS mRNA is thought to occur in at least two different conformations, one being a more closed structure with the translation initiation region base paired to an upstream internal antisense element, and the other being a more open and translationally competent structure. The translation-stimulating factors Hfq, HU, and DsrA RNA can bind to rpoS mRNA (indicated by broken heavy lines) and together probably drive it into the translationally competent structure. The other components shown are likely to act more indirectly (for further details, see the text).
- FIG. 4.
Role of RssB-ClpXP and putative signal input in the σS recognition and degradation pathway. The response regulator RssB is an essential, specific, and direct σS recognition factor. RssB delivers σS to the ClpXP protease, where σS is unfolded and completely degraded whereas RssB is released. σS binding requires RssB phosphorylation, but it is unclear whether the catalytic cycle of RssB involves obligatory dephosphorylation during release and subsequent rephosphorylation. Stress signals may affect (i) the phosphorylation of RssB and therefore RssB-σS complex formation; (ii) the cellular level of RssB (which in growing cells is rate limiting for σS proteolysis); (iii) the synthesis of σS such that RssB becomes titrated on σS overproduction; (iv) σS association with RNA polymerase core enzyme, which protects against binding by RssB; and (v) the function of the ClpXP protease itself (see the text for details). However, the molecular details of the stress signal input pathways involved are still largely unknown.
