END-PRODUCT INHIBITION OF THREONINE DEAMINASE OF STREPTOMYCIN MUTANTS OF ESCHERICHIA COLI K-12

Abstract

In the presence of end product (L-isoleucine), the biosynthetic threonine deaminase in extracts of streptomycin mutants of Escherichia coli K-12 gave a sigmoid response to increasing substrate (L-threonine) concentration. The response to inhibitor at constant substrate concentration was similar. However, the mutants showed quantitative differences in the "threshold" concentration of effector (substrate or inhibitor) required to produce the response. The smallest threshold was observed with the wild-type (streptomycin-sensitive) strain and the largest threshold with the streptomycin-resistant (indifferent) strain. The streptomycin-dependent strain was intermediate. The differences in response to effector may be caused by structural differences at the sites of bonding between subunits of the enzyme protein.These observations on feedback regulation, together with previous observations on repression in streptomycin mutants, indicate that the effects of the antibiotic are directed towards enzymes which perform a regulatory function.