This family consists of several ROS/MUCR transcriptional regulator proteins. The ros chromosomal gene is present in octopine and nopaline strains of Agrobacterium tumefaciens as well as in Rhizobium meliloti. This gene encodes a 15.5-kDa protein that specifically represses the virC and virD operons in the virulence region of the Ti plasmid [1] and is necessary for succinoglycan production [2]. Sinorhizobium meliloti can produce two types of acidic exopolysaccharides, succinoglycan and galactoglucan, that are interchangeable for infection of alfalfa nodules. MucR from Sinorhizobium meliloti acts as a transcriptional repressor that blocks the expression of the exp genes responsible for galactoglucan production therefore allowing the exclusive production of succinoglycan [3].

1: The virC and virD operons of the Agrobacterium Ti plasmid are regulated by the ros chromosomal gene: analysis of the cloned ros gene. Cooley MB, D'Souza MR, Kado CI; J Bacteriol 1991;173:2608-2616. PMID:2013576

2: Molecular analysis of the Rhizobium meliloti mucR gene regulating the biosynthesis of the exopolysaccharides succinoglycan and galactoglucan. Keller M, Roxlau A, Weng WM, Schmidt M, Quandt J, Niehaus K, Jording D, Arnold W, Puhler A; Mol Plant Microbe Interact 1995;8:267-277. PMID:7756693

3: MucR is necessary for galactoglucan production in Sinorhizobium meliloti EFB1. Martin M, Lloret J, Sanchez-Contreras M, Bonilla I, Rivilla R; Mol Plant Microbe Interact 2000;13:129-135. PMID:10656595