L-Rhamnose metabolism in Pichia stipitis and Debaryomyces polymorphus

The pathway for the breakdown of L-rhamnose by the yeast Pichia stipitis NRC 5568 was shown to involve nonphosphorylated intermediates and to produce pyruvate and L-lactaldehyde. The activities of the enzymes and the nature of several intermediates were determined. The enzymes involved are L-rhamnose dehydrogenase, L-rhamnonate dehydratase, and 2-keto-3-deoxy-L-rhamnonate aldolase. This pathway was found to be inducible by L-rhamnose and repressed by D-glucose. These enzymes were also present in a mutant of P. stipitis (PR1) resistant to catabolite repression and in Debaryomyces polymorphus 1747. Cell-free extracts of P. stipitis and D. polymorphus grown in L-rhamnose as sole carbon source were found to contain NAD+-dependent aldehyde dehydrogenase activities.Key words: Pichia stipitis, enzyme induction, oxidative pathway, catabolite repression.

Nuclear behaviour accompanying ascus formation in Debaryomyces polymorphus

Nuclear behaviour in growing, dividing, and ascospore-forming cells of a strain of Debaryomyces polymorphus, a member of the "Torulaspora group" of yeasts, has been studied by light microscopy of fixed Giemsa-stained preparations. Many of the images seen were compatible with the suggestion, advanced by certain earlier students, that meiosis in this type of yeast is preceded by a process of self-diploidization involving the nucleus of a bud that, despite its small size, is already separated from the parent cell by a cross wall. Diploidization, in this view, is achieved by the return of the bud nucleus to the parent cell via a channel in the cross wall. The bud nucleus next fuses with the nucleus of the parent cell. Self-diploidization in D. polymorphus is thus achieved in the guise of heterogamous conjugation. This in turn is followed by meiosis. A lesser number of cell associations suggestive of isogamous conjugation has been encountered also.