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I N T R O D U C T I O N All the Salmonella typhimurium mutants resistant to the methionine analogue norleucine described so far are also resistant to the analogues ethionine and a-methylmethionine. They can be classed either as metA(fbr) mutants with a homoserine-0-transsuccinylase, the first enzyme specific to methionine biosynthesis, resistant to feedback inhibition by methionine or a-methylmethionine, or as metK mutants with defects in S-adenosylmethionine (SAM) synthetase leading to abnormalities in methionine repression control (Lawrence, Smith & Rowbury, 1968; Chater & Rowbury, 1970; Hobson & Smith, 1973). A recently isolated norleucine-resistant mutant of a cysteine and thymine auxotroph, cysJ538 thyA, was found to be sensitive to ethionine. This mutant, provisionally designated cysJ538 thyA Nol-r, was studied in the hope that it might be a new type of methionine regulatory mutant.

A Norleucine-resistant Mutant of Salmonella typhimurium with a Possible Defect in Valine Uptake or Regulation