The regulation of synthesis of leucine biosynthetic enzymes in Neurospora.

Abstract

The elements of the regulatory mechanism apparently are (1) the repression of synthesis of the 1st enzyme of the pathway, the synthetase, as a function of endogenous leucine concentration, (2) the induced synthesis of subsequent enzymes of the pathway as a function of the concentration of [alpha]-IPM, the product of the first reaction, and (3) the regulation of inducer synthesis by feedback inhibition. The central role played by feedback Inhibition In the adjustment of the relative concentrations of the 3 enzymes is Illustrated by the drastic alterations in the levels of enzyme production obtained during growth of a feedback negative fluoro-leucine-resistant mutant and by a wild-type strain grown In the presence of the antirepressor 4-azaleucine. Apparently 2 different regulatory substances are Involved in the regulation of function of the leu cistrons. The 1st, a repressor substance, probably derived in part from leucine, is presumed to be analogous to repressors in bacterial systems and to act specifically by regulating either transcription or translation of theleu-4 clstron. The 2nd, and inducer, [alpha]-IPM, Is apparently involved In the regulation of function of the 3 unlinked cistrons that specify the synthesis of the isomerase and dehydrogenase.