REGULATION OF PROTEOLYSIS OF THE STATIONARY-PHASE SIGMA-FACTOR RPOS

RpoS, the stationary-phase sigma factor of Escherichia coli, is respon sible for increased transcription of an array of genes when cells ente r stationary phase and under certain stress conditions. RpoS is rapidl y degraded during exponential phase and much more slowly during statio nary phase; the resulting changes in RpoS accumulation play an importa nt role in providing differential expression of RpoS-dependent gene ex pression, It has previously been shown that rapid degradation gradatio n of RpoS during exponential growth depends on RssB (also called SprE and MviA), a protein with homology to the family of response regulator s, and on the ClpXP protease, We find that RssB regulation of proteoly sis does not extend to another ClpXP substrate, bacteriophage lambda O protein, suggesting that RssB acts on the specific substrate RpoS rat her than on the protease, In addition, the activity of RpoS is down-re gulated by RssB when degradation is blocked, In cells blocked for RpoS degradation by a mutation in clpP, cells devoid of RssB show a four- to fivefold-higher activity of an RpoS-dependent reporter fusion than cells overproducing RssB, Therefore, RssB allows specific environmenta l regulation of RpoS accumulation and may also modulate activity, The regulation of degradation provides an irreversible switch, while the r egulation of activity may. provide a second, presumably reversible lev el of control.