This is a family of proteins that are activated by PhoP. PhoP protein controls the expression of a large number of genes that mediate adaptation to low Mg2+ environments and/or virulence in several bacterial species. YbrL is proposed to be acting in a loop activity with PhoP and PrmA analogous to the multicomponent loop in Salmonella where the PhoP-dependent PmrD protein activates the regulatory protein PmrA, and the activated PmrA then represses transcription from the PmrD promoter which harbours binding sites for both the PhoP and PmrA proteins. Expression of YrbL is induced in low Mg2+ in a PhoP-dependent fashion and repressed by Fe3+ in a PmrA-dependent manner [1].

1: Dissecting the PhoP regulatory network of Escherichia coli and Salmonella enterica. Zwir I, Shin D, Kato A, Nishino K, Latifi T, Solomon F, Hare JM, Huang H, Groisman EA; Proc Natl Acad Sci U S A. 2005;102:2862-2867. PMID:15703297