A new member of MocR/GabR-type PLP-binding regulator of d-alanyl-d-alanine ligase in Brevibacillusbrevis

The Brevibacillusbrevis BBR47_28440 gene (referred to as ddlR) encodes an MocR/GabR family transcriptional regulator consisting of an N-terminal helix-turn-helix DNA binding domain and a C-terminal aminotransferase-like domain. The ddlR gene is located just upstream of the d-alanyl-d-alanine ligase gene (ddl) in the B.brevis genome, and these two genes form an operon. Gel-shift assays indicated that purified DdlR binds specifically to the DNA region that includes putative -35 and -10 regions of the ddlR promoter. A 6-bp inverted repeat that overlaps the -10 region of the ddlR promoter was found to be important for the binding. Invivo reporter assays confirmed that DdlR is an activator of the ddlR-ddl operon. Spectroscopic analyses indicated that purified DdlR is a pyridoxal 5-phosphate binding transcriptional regulator that has dipeptide binding ability for d-alanyl-d-alanine, the enzymatic product of Ddl, and glycylglycine. DdlR is capable of forming a dipeptide-pyridoxal 5-phosphate external aldimine, but it lacks aminotransferase activity. Bioinformatic analyses suggest that DdlR-mediated transcriptional regulation of ddlR and ddl may occur in multiple bacterial systems such as Actinobacteria and Bacillus species.