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syrM Is Involved in the Determination of the Amount and Ratio of the Two Forms of the Acidic Exopolysaccharide EPSI in Rhizobium meliloti

¹Institute of Genetics; ²Institute of Biophysics, Biological Research Center, Hungarian Academy of Sciences, Szeged P.O. Box 521, H-6701 Hungary; ³Institut des Sciences Végétales, C.N.R.S., Gif-sur-Yvette, France; ⁴Department of Plant Physiology, József Attila University, Szeged, P.O. Box 654, Hungary

For the invasion of root nodule cells of alfalfa by its symbiotic partner Rhizobium meliloti, the bacterial succinoglycan (EPSI) is required. Nitrogen starvation of R. meliloti results in a considerable increase of EPSI production. We have shown that the presence of nitrogen also altered the ratio of high and low molecular weight (HMW and LMW, respectively) forms of EPSI, favoring the production of LMW molecules. The transcription of two exo genes (exoF, exoP) was found to be under the control of combined nitrogen. In contrast, exoH and exoK were expressed independently of the nitrogen level. ntrC and syrM, regulatory genes for the nitrogen control of nod gene expression, 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 were 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 increased level of EPSI and the accumulation of the HMW fraction in nitrogen-containing 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 samples of the syrM mutant suggest a regulatory role for syrM in exoK expression and a contribution of exoK to the accumulation of the LMW form of EPSI.