THE REGULATION OF EXOPOLYSACCHARIDE PRODUCTION IS IMPORTANT AT 2 LEVELS OF NODULE DEVELOPMENT IN RHIZOBIUM-MELILOTI

We show that two exopolysaccharide overproducing Tn5 mutants of Rhizob ium meliloti, exoR and exoS, have distinct symbiotic defects. While th e exoR mutant is unable to colonize nodules, the exoS mutant retains t hat ability but varies in its ability to produce nitrogen-fixing nodul es. We correlate these defects with different degrees of exopolysaccha ride overproduction and growth impairment. We further show that the ex oR mutant is able to enter developing infection threads but is unable to invade nodule cells. The exoR mutant gives rise to spontaneous pseu dorevertants containing second-site suppressor mutations that decrease exopolysaccharide synthesis. These pseudorevertants form nitrogen-fix ing nodules. Although the suppressor mutations have the opposite effec t on exopolysaccharide production compared to the exoS::Tn5 mutation, they consistently map to the exoS::Tn5 region and belong to the same g enetic complementation group as defined by transposon insertion mutati ons. The effect of the suppressor mutations on exopolysaccharide produ ction is correlated with effects on the expression of exo genes involv ed in exopolysaccharide synthesis. Finally, we provide evidence that t he exoR gene is not required for the regulation of exopolysaccharide s ynthesis by ammonia.