Inhibits RpoS proteolysis by regulating RssB activity, thereby increasing the stability of the sigma stress factor RpoS during magnesium starvation. May also be involved in the early steps of isoprenoid biosynthesis, possibly through its role as RssB regulator. By magnesium and calcium starvation, via the transcriptional regulator PhoP. Belongs to the IraM/RssC family.