Jason Terpolilli

Exopolysaccharides (EPS's) play an important role in forming and extending infection threads in the symbiosis between Medicago and Sinorhizobium. S. meliloti 1021 (Sm1021) is able to produce one type of EPS (succinoglycan or EPS I) but not another (galactoglucan or EPS II), due to the presence of an insertion sequence (ISRm2011-1) in expR. ExpR forms part of the SinlR quorum sensing system and is a regulator of galactoglucan synthesis. Previous work in our laboratory determined that under N-limited conditions, Sm1021 was poorly effective at fixing N with the model indeterminate legume Medicago truncatula, while two other mucilaginous strains, S. medicae WSM419 and S. melitoti WSM1022, were significantly more effective on this host. While the expR status of WSM1022 is unknown, WSM419 possesses an intact expR gene. These data indicate that the interrupted expR gene in Sm1021 might account for the reduced effectiveness of this strain on the model legume.