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Applied and Environmental Microbiology, June 2005, p. 3228-3234, Vol. 71, No. 6
0099-2240/05/$08.00+0     doi:10.1128/AEM.71.6.3228-3234.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

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

Fermentation & Biotechnology Laboratories, Ajinomoto Co. Inc., 1-1 Suzuki-cho, Kawasaki-ku, Kawasaki-shi 210-8681, Japan

Received 29 September 2004/ Accepted 15 December 2004

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 -methylmethionine-resistant mutants, and these mutations were found to encode feedback-resistant enzymes in a ¹⁴C-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.

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