Escherichia coli and Salmonella typhimurium are clustered in one region of the genome near m t E and in E. coli are organized into two or three operons The expression of these operons is under the control of isoleucine, valine and leucine (FREUNDLICH, BURNS and UMBARGER 1962). Leucine biosynthesis in Salmonella is effekted by four enzymes, the structures of three of which are determined by four adjacent cistrons (BURNS, UMBARGER and GROSS 1963; MARGOLIN 1963). The expression of these genes in terms of enzyme levels is governed by the intracellular concentration of leucine and requires the functioning of several genetic elements. This conclusion was reached from studies on the coordinate synthesis of leucine enzymes (BURNS et d. 1966) and from an analysis of mutants isolated by resistance to a leucine analog, 5',5',5'-trifluoro-~~-leucine ( RENNERT and ANKER 1963; CALVO, FREUNDLICH and UMBARGER 1969). Many of the strains which are resistant to this analog are unable to regulate leucine production and as a result, overproduce leucine. One class of such mutants has high, constitutive leucine enzyme levels and the mutant sites of these strains lie in a region adjacent to the leucine structural cistrons. A second class has constitutive levels of enzymes involved in leucine, valine and isoleucine biosynthesis and the mutant sites involved are unlinked by transduction to the leucine operon The purpose of this communication is to identify a locus for the latter class, fir, which is located between purE and gal on the Salmonella genome.
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Alexander et al. (1969) studied this question.