Stringent Expression Control of Pathogenic R-body Production in Legume Symbiont Azorhizobium caulinodans

R bodies are insoluble large polymers consisting of small proteins encoded by genes and are coiled into cylindrical structures in bacterial cells. They were first discovered in species, which are obligate endosymbionts of paramecia. confers a killer trait on the host paramecia. R-body-producing symbionts are released from their host paramecia and kill symbiont-free paramecia after ingestion. The roles of R bodies have not been explained in bacteria other than ORS571, a microsymbiont of the legume , carries a operon containing four genes that are regulated by the repressor PraR. Herein, deletion of the gene resulted in R-body formation and death of host plant cells. The gene in the operon encodes an activator. Three PraR binding sites and a RebR binding site are present in the promoter region of the operon. Expression analyses using strains with mutations within the PraR binding site and/or the RebR binding site revealed that PraR and RebR directly control the expression of the operon and that PraR dominantly represses expression. Furthermore, we found that the operon is highly expressed at low temperatures and that 2-oxoglutarate induces the expression of the operon by inhibiting PraR binding to the promoter. We conclude that R bodies are toxic not only in paramecium symbiosis but also in relationships between other bacteria and eukaryotic cells and that R-body formation is controlled by environmental factors. species, which are obligate endosymbiotic bacteria of paramecia, produce R bodies, and R-body-producing endosymbionts that are released from their hosts are pathogenic to symbiont-free paramecia. Besides species, R bodies have also been observed in a few free-living bacteria, but the significance of R-body production in these bacteria is still unknown. Recent advances in genome sequencing technologies revealed that many Gram-negative bacteria possess genes encoding R-body components, and interestingly, many of them are animal and plant pathogens. , a microsymbiont of the tropical legume , also possesses genes. In this study, we demonstrate that has ability to kill the host plant cells by producing R bodies, suggesting that pathogenicity conferred by an R body might be universal in bacteria possessing genes. Furthermore, we provide the first insight into the molecular mechanism underlying the expression of R-body production in response to environmental factors, such as temperature and 2-oxoglutarate.