Structure of a polysaccharide containing galactose and galacturonic acid from Rhizobium meliloti. Characterization and partial purification of a 2-O-methyltransferase.

The teichuronic acid type polysaccharide found in Rhizobium meliloti which is associated with sensitivity to phage 16B and is formed in the inner membranes from UDP-galactose and UDP-galacturonic acid (Ugalde, R. A., Coira, J. A., and Brill, W. J. (1986) J. Bacteriol. 168, 270-275) has been studied further. Results of acid hydrolysis, periodate oxidation, and borohydride reduction show that this polysaccharide contains the repetitive unit -galacturonosyl(1-3)galactosyl(1-4-). A soluble enzyme was found to catalyze the transfer of methyl groups from S-adenosylmethionine to position 2 of the galacturonosyl residue. The enzyme requires Mn2+ or Mg2+, its pH optimum is 8.2, and the apparent Km for S-adenosylmethionine is 2.7 microM. The teichuronic acid type polysaccharide bound to a trichloroacetic acid-insoluble cell residue is a substrate for the methyltransferase; however, the polysaccharide released from the trichloroacetic acid-insoluble portion by mild acid treatment is no longer methylated. Other soluble galacturonic acid-containing polysaccharides were not used as substrates. The methyltransferase and the polysaccharide acceptor are both found in R. meliloti strain 102F51. Spontaneously arising mutants resistant to phage 16B do not form teichuronic acid but are transferase-positive. Other strains of R. meliloti as well as Agrobacterium tumefaciens and Escherichia coli cells do not form teichuronate and have no transferase.