Characterization Of mocR, A GntR Transcriptional Regulator In Bradyrhizobium japonicum

Abstract

The GntR family is one of the most widely distributed and prolific groups of the helix-turn-helix (HTH) transcription factors. In particular, microorganisms that live in complex, fast-changing environments such as soil tend to have a larger aggregate of the gntR regulatory genes. Bradyrhizobium japonicum is a Gram-negative soil bacterium capable of forming nodules and fixing nitrogen when in a symbiosis with the leguminous soybean plant (Glycine max). Although these metabolite-responsive gntR genes have been found to be involved in many cellular processes, little is known about their role in the B. japonicum-soybean symbiosis. The blr6977 gene (mocR), one of 35 gntR genes in B. japonicum, was investigated by generating a knock-out mutant strain. The mocR mutant strain reached a higher saturation of optical density than the wild type only when grown in minimal media. Motility tests using 0.3% minimal medium agar also revealed enhanced motility by the mocR mutant compared to that of the wild type. Nodulation experiments were conducted in order to determine a nodulation phenotype of the mutant strain. The outcome indicated the mocR mutant was deficient in the number of nodules compared to the wild type, and it resulted in delayed nodulation. An electrophoretic mobility shift assay (EMSA) revealed protein-DNA interaction using purified MocR protein and a probe containing the promoter region upstream of blr6977, indicating the possible autoregulation of the mocR gene.