REGULATORY DEFICIENCIES IN MUTANTS OF HYDROGENOMONAS H-16 RESISTANT TO TRIFLUOROLEUCINE

1. dl-5′,5′,5′-trifluoroleucine exerts a bacteriostatic action on Hydrogenomonas H 16; growth is completely suppressed by the antimetabolite at concentrations higher than 5x10-5 M. After treatment by 1-methyl-3-nitro-1-nitrosoguanidine mutants were selected which were resistant to 10-3M trifluoroleucine. Four different types of mutants were recognized: (i) The mutant r 2 is constitutively derepressed for the formation of the enzyme α-isopropylmalate synthetase; the enzyme activity is increased elevenfold. (ii) In the mutant s 8 the enzyme α-isopropylmalate synthetase is insensitive to endproduct inhibition by l-leucine. (iii) Several mutants are constitutively derepressed for the formation of acetohydroxy acid synthase (3 to 23 fold increase of enzyme activity). (iv) In the mutant s 16 the enzyme acetohydroxy acid synthase is insensitive to endproduct inhibition by l-valine. 2. Prototrophic revertants of isoleucine or isoleucine-valine requiring auxotrophic mutants of H 16 which produce acetohydroxyacid synthase constitutively or which produce an enzyme with altered valine sensitivity, proved to be resistant to 10-3 M trifluoroleucine. 3. The isolation of mutants carrying regulatory defects in the regulation of the valine-isoleucine biosynthetic pathway by means of trifluoroleucine is due to an unusual property of the α-isopropylmalate synthetase of Hydrogenomonas H 16. l-valine is not only related to the leucine pathway as a precursor, but acts as a positive effector for the first enzyme of the leucine pathway. In this way the overproduction of valine is followed by an overproduction of leucine which finally conferes resistance to the bacteriostatic effect of trifluoroleucine. 4. In Escherichia coli and Pseudomonas aeruginosal-valine does not act antagonistically to leucine at the enzyme α-isopropylmalate synthetase. In Micrococcus denitrificansl-valine acts as a positive effector as it does in Hydrogenomonas H 16. © 1969 Springer-Verlag.