Assimilation and Toxicity of Exogenous Amino Acids in the Methane-oxidizing Bacterium Methylococcus capsulatus

Summary: Of 21 amino acids tested, only L- and D-threonine, L-phenylalanine, L-histidine, L-tyrosine and L-homoserine inhibited exponential growth of Methylococcus capsulatus at 1.0 mM. Inhibition by L-threonine was relieved by L-methionine, L-homoserine, L-alanine and L-valine, but not by L-lysine, 2, 6-diaminopimelic acid or L-arginine. 14C-labelled methane, L-aspartate, L-threonine, L-homoserine, L-glutamic acid, L-phenylalanine and L-tryptophan were all assimilated. The results suggested that the branched pathway for threonine, isoleucine, methionine and lysine synthesis from aspartate is functional. An explanation of threonine-inhibition in terms of an interference with end-product regulation of this pathway is proposed.