pSym

pSym is a term used in microbiology to describe a class of large extrachromosomal replicons, or megaplasmids, found in rhizobia that carry genes essential for establishing legume symbioses. In many Sinorhizobium and Rhizobium species, the major pSym replicons are designated pSymA and pSymB. These elements are separate from the bacterial chromosome and can greatly influence a strain’s ability to nodulate specific hosts and fix nitrogen.

Genetic content commonly associated with pSym includes nodulation genes that initiate and regulate nodulation, regulatory genes responsive to plant signals such as flavonoids, and various genes linked to nitrogen fixation, exopolysaccharide production, and other symbiotic traits. pSymA is frequently highlighted for containing key nod and nif genes, which determine host range and efficiency, while pSymB contributes additional traits that support the symbiotic lifestyle. The exact gene sets vary among species and strains, reflecting substantial genetic diversity and modular evolution.

Role in symbiosis and host interaction is central to pSym function. These plasmids enable rhizobia to recognize legume hosts, initiate nodulation, and establish nitrogen-fixing root nodules. Their gene content influences host specificity, infection efficiency, and nodulation timing, and they are often regulated by plant-derived signals.

Mobility and evolution are notable features. pSym replicons can be transferred between strains by conjugation, contributing to horizontal gene transfer and rapid evolution of symbiotic capabilities. They are subject to rearrangements and mosaic genetic structures, reflecting adaptation to different hosts and environments.

In summary, pSym refers to symbiotic megaplasmids in rhizobia that carry essential and accessory genes for nodulation and nitrogen fixation, with pSymA and pSymB being the best-characterized representatives in several Sinorhizobium and Rhizobium species.