SUMMARY: Forty-six methionineless mutants of were arranged in three phenotypic groups according to their growth responses to potential precursors of methionine. The results of syntrophism tests led to the recognition of two more phenotypic groups and permitted the arrangement in sequence of the metabolic steps in which mutants of each of the five groups were deficient. Transduction experiments indicated that each of these groups comprised mutants whose sites of mutation were closely linked within a complex locus. Attempts to map the sites of mutation of 12 mutants within one locus were unsuccessful. One of the mutants, probably a deletion, failed to recombine with the other 11 and behaved differently from them in linked transduction. A group of three and one pair of the 11 mutants could not be separated by recombination. Linkage was detected by transduction between only 2 of the 5 loci; these were concerned with non-sequential steps in the biosynthesis of methionine. No linkage was detected between the methionine loci and any of a number of other loci, including those controlling the biosyntheses of cysteine and tryptophan. These results were only partly in accordance with a previously suggested linkage map.


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