Genome-wide transcriptional profiling of Azorhizobium caulinodans ORS571 (CBX88). unidentified

A whole genome sequence of the endosymbiotic bacterium Azorhizobium caulinodans ORS571, which forms nitrogen-fixing nodules on the stems and roots of Sesbania rostrata, was recently determined. It revealed that the sizes of the genome and symbiotic island were 5.4 Mb and 86.7 kb, respectively, and these sizes are the smallest among the sequenced rhizobia. In the present study, a whole genome microarray of A. caulinodans was constructed and transcriptomic analyses were performed on free-living cells grown in rich and minimal media, and bacteroids isolated from stem nodules. The transcriptional profiling showed that the genes involved in sulfur uptake and metabolism, acetone metabolism, and biosynthesis of exopolysaccharide were highly expressed in bacteroids as compared to free-living cells. Some mutants having Tn5 transposon within these up-regulated genes were obtained as nodule-deficient mutants in our previous study. A transcriptomic analysis was also performed on free-living cells in a minimal media supplemented with a flavonoid, naringenin, which is one of the most efficient inducers of A. caulinodans nod genes. Only 18 genes were up-regulated by addition of naringenin, suggesting that regulatory mechanism responding to flavonoid could be simple in A. caulinodans. The combination of the genome-wide transcriptional profiling and our previous genome-wide mutagenesis study has revealed new aspects of nodule formation and maintenance.