Influence of exopolysaccharide composition and N-source limitation on infectivity of Bradyrhizobium japonicum

QUELAS, J.I.; LÓPEZ GARCÍA, S.L.; CASABUONO, A.; ALTHABEGOITI, M.J.; MONGIARDINI, E.; PÉREZ GIMÉNEZ, J.; COUTO, A.; LODEIRO, A.R.

Iguazú, Argentina

Congreso; XL Reunión Anual SAIB; 2004

Sociedad Argentina de Investigación Bioquímica y Biología Molecular

The exopolysaccharide (EPS) of B. japonicum plays a key role in the rate of bacterial penetration (infectivity) of soybean roots for the development of N2-fixing nodules. Since the composition and amount of these molecules may change according to the bacterial growth conditions, it is important to assess these effects on infectivity. Here we analyzed the EPS composition in the wild-type strain USDA 110 and in an exoB mutant, both grown in defined N-sufficient or N-limiting media, and with mannitol or malate as sole C-source. The samples were obtained by ethanol precipitation of culture supernatants and TFA-hydrolysis followed by HPAEC-PAD analysis. Both strains produced more EPS (mg sugar/mg cell protein) under N-starvation. However, no changes were observed in the EPS composition of each strain when grown with different C-sources or N-starvation. The wild-type EPS was composed of 1 Glu : 0.5 (OMe/OAc)Gal : 0.5 (OAc)GalA : 0.17 Man. The mutant EPS lacked Gal, as expected from the absence of UDP-4-Glu epimerase activity, but contained GalA. Although the exoB mutant was less infective than the wild-type with both C-sources, each strain was more infective when grown under N-starvation in relation with the same bacteria from N-sufficient broths