PmrB forms a two-component system (TCS) with PmrA that allows Gram-negative bacteria to survive the cationic antimicrobial peptide polymyxin G [1]. The TCS is linked to another one via the polymyxin resistance protein PmrD. PmrD is the first protein identified to mediate the connectivity between the two TCSs. It binds to the N terminal domain of the PmrA response regulator which prevents its dephosphorylation, thereby promoting the the transcription of genes involved in polymyxin resistance [2].

1: First structure of the polymyxin resistance proteins. Fu W, Yang F, Kang X, Zhang X, Li Y, Xia B, Jin C; Biochem Biophys Res Commun. 2007;361:1033-1037. PMID:17686460

2: Connecting two-component regulatory systems by a protein that protects a response regulator from dephosphorylation by its cognate sensor. Kato A, Groisman EA; Genes Dev. 2004;18:2302-2313. PMID:15371344