Control of isoleucine, valine, and leucine biosynthesis. VI. Effect of 5',5',5'-trifluoroleucine on repression in Salmonella typhimurium.

The leucine analogue 5',5',5',-trifluoroleucine (fluoroleucine) replaced leucine for repression of the isoleucine-valine biosynthetic enzymes in Salmonella typhimurium. In contrast, the analogue had no effect on derepression of the leucine biosynthetic enzymes in leucine auxotrophs grown on limiting amounts of leucine. The effect of fluoroleucine on repression appeared to be specific for leucine since derepression of the isoleucine-valine enzymes due to an isoleucine or valine limitation was not affected by the analogue. The prevention of derepression by fluoroleucine was probably due to repression and not to the formation of false proteins, since the analogue had no effect on the derepression of a number of enzymes unrelated to the isoleucine-valine pathway. Fluoroleucine was able to attach to leucine transfer ribonucleic acid (tRNA) as evidenced by the ability of the analogue to protect about 70% of leucine tRNA from oxidation by periodate. We propose that the differential effects of fluoroleucine on repression are due to differences in the ability of the analogue to bind to the various species of leucine tRNA.