Dissecting Roles Of A LuxR Family Transcriptional Regulator,ExpR In Sinorhizobium Meliloti And Functions Of Enzymes(GGDEF And EAL Domain Containing Proteins) Responsible For Synthesis And Degradation Of C-di-GMP

About80%of the atmosphere is the nitrogen, but it can not be used by higher animals and plants directly. Biological nitrogen fixation affords the most part of nitrogen input.It is a hotspot in current study. Establishing this symbiosis requires a response of molecular signals, gene expression and nutrient exchange between the legumes and the rhizobia. Therefore, analyse the rhizobia and the relationship between rhizobia and legumes will be the main goal of our study, which has important scientific value in reducing agricultural production costs, maintaining the sustainable development of agriculture and environment.A new bacterial secondary messenger, bis (3’-5’)-cyclic dimeric guanosine monophosphate (c-di-GMP), is usually synthesized or decomposed by proteins containing a GGDEF or EAL domain. These proteins often act as a cyclase or phosphodiesterase of c-di-GMP, and their genes are distributed among almost all bacteria according to known genomic DNA sequences. However, the systematic identification of GGDEF and EAL protein genes remains unclear in rhizobia, soil bacteria that interact with compatible legumes to form nitrogen-fixing nodules. In this study,19putative GGDEF and EAL protein genes were identified in a model rhizobium, Sinorhizobium meliloti, by bioinformatic analysis (encoding5GGDEF proteins,4EAL proteins, and10GGDEF and EAL double-domain proteins). Null mutants of14genes were constructed through systematic plasmid insertion. All14gene mutants showed deficient growth in minimal medium and defective motility, and11gene mutants produced a lot more exopolysaccharide and displayed less competitive nodulation on the host plant, alfalfa. We also find that these mutants had different capacity to utilize sugars and amino acids as carbon and nitrogen sources respectively. Our results suggest that GGDEF and EAL proteins may play different roles in the control of S. meliloti physiology, although they contain conserved catalytic (GGDEF or EAL) domains. Our finding also implies that c-di-GMP may play an important role in the interactions between this rhizobium and its host plants to establish efficient symbiosis.Additionally, S. meliloti strain carrying an intact expR gene induced the same efficiently nitrogen-fixing nodules on Medicago sativa as the wild type strain; however, it only induced lower efficient nodules on M. truncatula. And a now mutation locus was proposed, and the detail will be elucidated in the future.