SUMMARY: In mutants of , which are defective in glucose repression of several enzymes, growth is inhibited if maltose is present in the medium. After adding [C]maltose to cultures growing with ethanol, maltose metabolism was followed in both mutant and wild-type cells. The amount of radioactivity incorporated was much higher in than in wild-type cells. Most of the radioactivity in cells was located in the low molecular mass fraction. Pulse-chase experiments showed that 2 h after addition of maltose, cells hydrolysed maltose to glucose, which was partially excreted into the medium. P-NMR studies gave evidence that turnover of sugar phosphates was completely abolished in cells after 2 h incubation with maltose. C-NMR spectra confirmed these results: unlike those for the wild-type, no resonances corresponding to fermentation products (ethanol, glycerol) were found for cells, whereas there were resonances corresponding to glucose. Although maltose is taken up by proton symport, the internal pH in the mutant did not change markedly during the 5 h after adding maltose. The intracellular accumulation of glucose seems to explain the inhibition of growth by maltose, probably by means of osmotic damage and/or unspecific -glycosylation of proteins. Neither maltose permease nor maltase was over-expressed, and so these enzymes were not the cause of glucose accumulation. Hence, the coordination of maltose uptake, hydrolysis to glucose and glycolysis of glucose is not regulated simply by the specific activity of the catabolic enzymes involved. The results indicate that there is an unknown regulatory mechanism, under control of , which coordinates glycolytic flux and maltose uptake. Furthermore, the excretion of accumulated glucose into the medium gives clear evidence that at least one glucose carrier in acts passively and transports glucose in both directions.


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