Effects of Deregulation of Methionine Biosynthesis on Methionine Excretion in Escherichia coli

Abstract

ABSTRACT Several regulators of methionine biosynthesis have been reported in Escherichia coli , which might represent barriers to the production of excess l -methionine (Met). In order to examine the effects of these factors on Met biosynthesis and metabolism, deletion mutations of the methionine repressor ( metJ ) and threonine biosynthetic ( thrBC ) genes were introduced into the W3110 wild-type strain of E. coli . Mutations of the metK gene encoding S -adenosylmethionine synthetase, which is involved in Met metabolism, were detected in 12 norleucine-resistant mutants. Three of the mutations in the metK structural gene were then introduced into metJ and thrBC double-mutant strains; one of the resultant strains was found to accumulate 0.13 g/liter Met. Mutations of the metA gene encoding homoserine succinyltransferase were detected in α-methylmethionine-resistant mutants, and these mutations were found to encode feedback-resistant enzymes in a 14 C-labeled homoserine assay. Three metA mutations were introduced, using expression plasmids, into an E. coli strain that was shown to accumulate 0.24 g/liter Met. Combining mutations that affect the deregulation of Met biosynthesis and metabolism is therefore an effective approach for the production of Met-excreting strains.