Differential degradation of Escherichia coli sigma(32) and Bradyrhizobium japonicum RpoH factors by the FtsH protease

The Escherichia coli heat shock sigma factor sigma(32) (RpoH) is rapidly de graded under non-stress conditions. The integrity of the DnaK chaperone mac hinery and the ATP-dependent FtsH protease are required for sigma(32) prote olysis in vivo. Bradyrhizobium japonicum expresses three sigma(32)-type tra nscription factors, RpoH(1), RpoH(2), and RpoH(3), which are functional in E. coli. We compared the stability of these sigma factors with E. coli sigm a(32) stability. In E. coli C600 (wild-type), the half-lives of sigma(32), RpoH(1), RpoH(2) and RpoH(3) were 30 s, 7 min, 4 min and 4 min, respectivel y. The first three proteins were stabilized in ftsH mutant backgrounds, ind icating that they are degraded by FtsH in the wild-type. Proteolysis of Rpo H(3) was FtsH-independent because this sigma factor was not stabilized in f tsH mutants. Interestingly, in a purified in vitro system, all four RpoH pr oteins were degraded by FtsH, indicating that in vivo protein degradation d epends on additional cellular factors. Rationally designed point mutations of sigma(32) and RpoH(1) suggested that the highly conserved RpoH box does not play a major role in conferring stability to RpoH factors. Presumably, several regions distributed along the primary sequence of the sigma factor are important for FtsH-mediated proteolysis. Finally, we provide evidence t hat proteolysis of RpoH factors in vivo depends on the DnaK machinery, irre spective of the protease involved.