Influence of the Carbon:Nitrogen Ratio of the Growth Medium on the Cellular Composition and the Ability of the Methylotrophic Yeast Hansenula polymorpha to Utilize Mixed Carbon Sources

Summary: The methylotrophic yeast Hansenula polymorpha was grown in a chemostat with a medium containing a mixture of glucose (C6) and methanol (C1) (87·8% C6: 12·2% C1, w/w) as sole carbon source and NH4 + as sole nitrogen source. At a constant growth rate (D 0·10 h-1) the influence of the carbon: nitrogen ratio (C:N) of the inflowing medium on the cellular and enzymic composition of the cells was studied. Three distinct growth regimes were recognized. A medium with a C:N ratio 31 resulted in nitrogen-limited growth (low protein but high carbohydrate content of the cells) and the cells metabolized only glucose. A transition growth regime was observed during growth on media with intermediate C:N ratios (12 31). When assessed from both substrate consumption and cellular composition, growth was double-substrate (carbon and nitrogen)-limited. In this transition growth regime, changes in carbon metabolism and the cellular and enzymic composition of the cells were found. With increasing C:N ratios in the growth medium a gradual repression of the synthesis of methanol-assimilating and dissimilating enzymes was found. This effect was most pronounced for alcohol oxidase, and as a consequence the cells switched from the utilization of the carbon substrate mixture to growth on glucose alone. The data presented suggest that the range within which double-substrate-limited growth can be expected is predictable from the composition of cells grown under single substrate limitation.