THE PRODUCT OF THE LEU-3 CISTRON AS A REGULATORY ELEMENT FOR THE PRODUCTION OF THE LEUCINE BIOSYNTHETIC ENZYMES OF NEUROSPORA

ABSTRACT A class of intracistronic (or closely linked) partial reversions of leu-3 mutations has been found to be conditionally constitutive with respect to the synthesis of isopropylmalate isomerase (specified by the leu-2 cistron) and β-isopropylmalate dehydrogenase (specified by the leu-1 cistron), two of the enzymes of leucine biosynthesis in Neurospora. The intermediate level of enzyme production by these leu-3cc mutants is independent of the obligatory inducer effector, α-isopropylmalate, but dependent upon the presence of the branched-chain amino acids, isoleucine, valine and leucine. The properties of leu-3+, leu-3 and leu-3cc in heterokaryons indicate that the transnuclear regulatory activity of the leu-3 product varies specifically as a function of available effector molecules. The information presented suggests that the leu-3 cistron is responsible not only for the production of a "positive" regulatory substance necessary for the expression of the leu-1 and leu-2 cistrons, but that it probably serves also a coordinating role in the expression of many of the genes involved in branched-chain amino acid metabolism.