Functional substitution of alternative homoserine acyltransferases in E. coli

Abstract

Homoserine transsuccinylase (HTS) is a bacterial enzyme that catalyzes the first unique step in methionine biosynthesis. Since methionine is an essential amino acid, enzymes in this pathway may be good targets for novel antibiotics. Remarkably, some bacteria use a genetically distinct enzyme, homoserine transacetylase (HTA), in the first unique step of methionine biosynthesis. We have obtained a strain of E. coli with an inactivated gene for HTS and used this to probe the ability of HTS and HTA enzymes from different organisms to functionally substitute for the E. coli protein. We have also used this strain to investigate the ability of mutants of the E. coli enzyme that have reduced catalytic activity to rescue growth. These experiments indicate that enzymes from other organisms poorly rescue growth or are unable to rescue growth. Mutants of the E. coli enzyme with an in vitro activity below 5% are unable to adequately rescue growth. The results of this study indicate that a minimal rate of methionine biosynthesis is required for E. coli growth in minimal media and that substitution of other homoserine acyltransferases is not well tolerated by the organism.