The response regulator RssB, a recognition factor for σ(S) proteolysis in Escherichia coli, can act like an anti-σ(S) factor

Abstract

s(S) (RpoS) is the master regulator of the general stress response in Escherichia coli. Several stresses increase cellular s(S) levels by inhibiting proteolysis of s(S), which under non-stress conditions is a highly unstable protein. For this ClpXP-dependent degradation, the response regulator RssB acts as a recognition factor, with RssB affinity for s(S) being modulated by phosphorylation. Here, we demonstrate that RssB can also act like an anti-sigma factor for s(S) in vivo, i.e. RssB can inhibit the expression of s(S)-dependent genes in the presence of high s(S) levels. This becomes apparent when (i) the cellular RssB/s(S) ratio is at least somewhat elevated and (ii) proteolysis is reduced (for example in stationary phase) or eliminated (for example in a clpP mutant). Two modes of inhibition of s(S) by RssB can be distinguished. The 'catalytic' mode is observed in stationary phase cells with a substoichiometric RssB/s(S) ratio, requires ClpP and therefore probably corresponds to sequestering of s(S) to Clp protease (even though s(S) is not degraded). The 'stoichiometric' mode occurs in clpP mutant cells upon overproduction of RssB to levels that are equal to those of s(S), and therefore probably involves binary complex formation between RssB and σ(S.) We also show that, under standard laboratory conditions, the cellular level of RssB is more than 20-fold lower than that of s(S) and is not significantly controlled by stresses that upregulate s(S). We therefore propose that antisigma factor activity of RssB may play a role under not yet identified growth conditions (which may result in RssB induction), or that RssB is a former antisigma factor that during evolution was recruited to serve as a recognition factor for proteolysis.