Comparative genomic analysis of Sinorhizobium meliloti LPU88: plasmid diversity and conjugative mechanisms

In this study, we present a comprehensive genomic and comparative analysis of Sinorhizobium meliloti strain LPU88, highlighting the structure, function, and evolutionary dynamics of its plasmids. The complete genome sequencing revealed five replicons: a chromosome, two megaplasmids (pSymA-like pSmeLPU88c and pSymB-like pSmeLPU88d), and two accessory plasmids (pSmeLPU88a and pSmeLPU88b). Furthermore, the genome of LPU88 harbored a rich repertoire of mobile genetic elements, diverse replication modules, and unique gene clusters, reflecting its dynamic architecture. Strain LPU88 contained diverse conjugation systems distributed across its plasmids. Comparative analyses with other S. meliloti and Sinorhizobium medicae strains demonstrated the heterogeneous distribution of conjugative and regulatory elements, indicating variable evolutionary pressures among these plasmids. Besides, the mobilization of the pSymA-like plasmid pSmeLPU88c was mediated by a mating pair formation system encoded on the accessory plasmid pSmeLPU88a, reflecting the intricate mechanisms and evolutionary dynamics of horizontal gene transfer mediated by plasmids in Sinorhizobium. By integrating genomic sequencing, functional annotation, and comparative approaches, this work establishes LPU88 as a valuable model strain for understanding plasmid diversity, horizontal gene transfer, and symbiotic efficiency in rhizobia.

Rhizobia are soil bacteria that establish symbiotic associations with legumes, converting atmospheric nitrogen into ammonia through biological nitrogen fixation, while the host provides nutrients. Among them, Sinorhizobium meliloti is one of the best-studied species. In this work, we compared the complete genomes of S. meliloti strains, including the laboratory model strain LPU88, with a particular focus on pSymA plasmids. Previous studies proposed that the pSymA plasmid could have been acquired through horizontal gene transfer. Analysis of their conjugation machinery revealed that all pSymA plasmids harbor a type II conjugation system, although in many cases the regulatory circuit required for activation was absent. In LPU88, we identified and characterized multiple conjugation systems, offering new insights into horizontal gene transfer in S. meliloti. Understanding these processes is essential for clarifying rhizobial evolutionary dynamics, improving the stability and efficiency of symbiotic interactions, and promoting their use as bioinoculants in sustainable agriculture.