LEUCINE BIOSYNTHESIS AND ITS REGULATION IN THE BASIDIOMYCETE RHODOSPORIDIUM TORULOIDES

Complementation tests and enzyme analyses separated 29 leucine auxotro phs of the Basidiomycete Rhodosporidium toruloides into three groups, each deficient in one of the leucine biosynthetic enzymes. The followi ng differences are suggested between the organization of the leucine p athway in R. toruloides and the Ascomycetes Saccharomyces cerevisiae; and Neurospora crassa: (1) isopropylmalate, the product of the first e nzymic reaction appears not to be an internal inducer of the later enz ymes of the pathway; this is consistent with the apparent lack of muta nts homologous to the leu3 class in N. crassa and S. cerevisiae; (2) a s in S. cerevisiae, but unlike N. crassa, isopropylmalate synthase is under the control of a general cross pathway control system; (3) unlik e S. cerevisiae, but like N. crassa, R. toruloides appears to possess only one gene encoding isopropylmalate synthase.