syrM is involved in the determination of the amount and ratio of the two forms of the acidic exopolysaccharide EPSI in Rhizobium meliloti

For the invasion of root nodule cells of alfalfa by its symbiotic partner R hizobium meliloti, the bacterial succinoglycan (EPSI) is required. Nitrogen starvation of R. meliloti results in a considerable increase of EPSI produ ction. We have shown that the presence of nitrogen also altered the ratio o f high and low molecular weight (HMW and LMW, respectively) forms of EPSI, favoring the production of LMW molecules, The transcription of two exo gene s (exoF, exoP) was found to be under the control of combined nitrogen. In c ontrast, exoH and exoK were expressed independently of the nitrogen level. ntrC and syrM, regulatory genes for the nitrogen control of nod gene expres sion, were shown to be involved in the regulation of EPSI production. In an syrM mutant both the amount and the ratio of the two main forms of EPSI we re altered, compared with the wild-type strain: a reduced level of EPSI and the dominance of the LMW form in nitrogen-starved cultures, while an incre ased level of EPSI and the accumulation of the HMW fraction in nitrogen-con taining samples was observed. An enhanced expression level of the exoK gene and the higher ratio of the LMW form to the total EPSI in nitrogen-free sa mples of the syrM mutant suggest a regulatory role for syrM in exoK express ion and a contribution of exoK to the accumulation of the LMW form of EPSI.