Control of isoleucine, valine and leucine biosynthesis VIII. Mechanism of growth inhibition by leucine in relaxed and stringent strains of Escherichia coli K-12

1. 1. Leucine was found to inhibit the growth of Escherichia coli K-12 and stringent strains after downshift transfer from nutrient broth to minimal medium. Leucine had little or no effect on the growth of E. coli W or relaxed and stringent strains of Salmonella typhimurium. 2. 2. Leucine inhibition was overcome by isoleucine or any isoleucine precursor formed after acetohydroxy acid synthetase. The lag in growth caused by leucine was correlated with repression of acetohydroxy acid synthetase. 3. 3. The growth of a mutant of E. coli K-12 that is genetically derepressed for acetohydroxy acid synthetase was not inhibited by leucine. 4. 4. The major factor in leucine inhibition appears to be the altered pattern of multivalent repression found in E. coli K-12 manifested primarily in the inability of isoleucine to regulate the formation of acetohydroxy acid synthetase.