The utilization of peptides and L- and D-amino acids by effective and ineffective strains of Rhizobium meliloti.

Abstract

Washed cells of three strains of Rhizobium meliloti, effective and ineffective in the fixation of atmospheric nitrogen, were found to utilize certain di- and tri-peptides and the L-, D-, and DL-forms of several amino acids as sole sources of nitrogen. The maximum growth attained on any one amino acid isomer was a function of pH and temperature. Inhibitory effects found in certain racemic mixtures were explained by mutual stereochemical interference of the enzymes attacking the L- and D-isomers. Histidine was notable in that it yielded the greatest amount of growth at all pH values (6.0, 7.0, 7.5) with all three organisms, including the ineffective strain which did not grow at pH 6.0 or 7.5 on any other acid. All strains were relatively acid sensitive, variable degrees of growth inhibition occurring at pH 6.0. Partial hydrolysis of the peptides occurred, but stimulatory effects were observed in many instances and explained on the basis of transpeptidation. Inhibition in media containing peptides of glycine plus D-leucine was traced to an antagonistic effect between the two constituent amino acids after liberation. Although the ineffective and effective strains of rhizobia were found to differ in several ways, the most interesting feature was the stimulation of the growth of the ineffective organism by D-leucylglycine.