Summary: The methylotrophic yeast sp. 2201 was grown in double carbon (glucose/methanol) limitation in a chemostat at a constant dilution rate and the regulation of the synthesis of enzymes involved in the assimilation of methanol and the breakdown of glucose was studied as a function of the composition of the glucose/methanol mixture in the inflowing medium. Enhanced synthesis of most of the enzymes taking part in the assimilation of C units took place despite the presence of glucose in the medium. Depending on the enzyme, 40 to 60% (w/w of total substrate) methanol in the mixture was enough to cause maximal induction and no further enhancement of their specific activities was observed during growth on mixtures containing higher methanol/glucose ratios. The only two enzymes involved in the assimilation of methanol not showing these characteristic patterns were ‘classical’ transketolase and transaldolase. Substrate fluxes through the individual steps of glucose and methanol metabolism have been calculated and compared with the experimentally determined specific activities of the enzymes concerned. The three enzymes whose specific activities were least in excess of the substrate flux calculated to be catalysed by them were transketolase, transaldolase and (possibly) dihydroxyacetone synthase.

The residual concentration of methanol was measured in the culture as a function of the composition of the supplied glucose/methanol mixture. During growth with mixtures the concentration of methanol was always lower than during growth with this substrate as the only carbon source. Possible ecological advantages accruing to a facultative methylotroph as a consequence of this behaviour are discussed.


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