Characterization of LgnR, an IclR family transcriptional regulator involved in the regulation of l-gluconate catabolic genes in Paracoccus sp. 43P

Abstract

Five genes encoding enzymes required for L-gluconate catabolism, together with genes encoding components of putative ABC transporters, are located in a cluster in the genome of Paracoccus sp. 43P. A gene encoding a transcriptional regulator in the IclR family, lgnR, is located in front of the cluster in the opposite direction. Reverse transcription PCR analysis indicated that the cluster was transcribed as an operon, termed the lgn operon. Two promoters, P(lgnA) and P(lgnR), are divergently located in the intergenic region, and transcription from these promoters was induced by addition of L-gluconate or D-idonate, a catabolite of L-gluconate. Deletion of lgnR resulted in constitutive expression of lgnA, lgnH and lgnR, indicating that lgnR encodes a repressor protein for the expression of the lgn operon and lgnR itself. Electrophoretic mobility shift assay and DNase I footprinting analyses revealed that recombinant LgnR binds to both P(lgnA) and P(lgnR), indicating that LgnR represses transcription from these promoters by competing with RNA polymerase for binding to these sequences. D-Idonate was identified as a candidate effector molecule for dissociation of LgnR from these promoters. Phylogenetic analysis revealed that LgnR formed a cluster with putative proteins from other genome sequences, which is distinct from those proteins of known regulatory functions, in the IclR family of transcriptional regulators. Additionally, the phylogeny suggests an evolutionary linkage between the L-gluconate catabolic pathway and D-galactonate catabolic pathways distributed in Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria and Actinobacteria.