In previous studies it was shown that deletion of the HXK2 gene in Saccharomyces cerevisiae yields a strain that hardly produces ethanol and grows almost exclusively oxidatively in the presence of abundant glucose. This paper reports on physiological studies on the hxk2 deletion strain on mixtures of glucose/sucrose, glucose/galactose, glucose/ maltose and glucose/ethanol in aerobic batch cultures. The hxk2 deletion strain co-consumed galactose and sucrose, together with glucose. In addition, co-consumption of glucose and ethanol was observed during the early exponential growth phase. In S. cerevisiae, co-consumption of ethanol and glucose (in the presence of abundant glucose) has never been reported before. The specific respiration rate of the hxk2 deletion strain growing on the glucose/ethanol mixture was 900 mmolemin x1 e(g protein) x1 , which is four to five times higher than that of the hxk2 deletion strain growing oxidatively on glucose, three times higher than its parent growing on ethanol (when respiration is fully derepressed) and is almost 10 times higher than its parent growing on glucose (when respiration is repressed). This indicates that the hxk2 deletion strain has a strongly enhanced oxidative capacity when grown on a mixture of glucose and ethanol.

In previous studies it was shown that deletion of the HXK2 gene in Saccharomyces cerevisiae yields a strain that hardly produces ethanol and grows almost exclusively oxidatively in the presence of abundant glucose. This paper reports on physiological studies on the hxk2 deletion strain on mixtures of glucose/sucrose, glucose/galactose, glucose/maltose and glucose/ethanol in aerobic batch cultures. The hxk2 deletion strain co-consumed galactose and sucrose, together with glucose. In addition, co-consumption of glucose and ethanol was observed during the early exponential growth phase. In S.cerevisiae, co-consumption of ethanol and glucose (in the presence of abundant glucose) has never been reported before. The specific respiration rate of the hxk2 deletion strain growing on the glucose/ethanol mixture was 900 micromol.min(-1).(g protein)(-1), which is four to five times higher than that of the hxk2 deletion strain growing oxidatively on glucose, three times higher than its parent growing on ethanol (when respiration is fully derepressed) and is almost 10 times higher than its parent growing on glucose (when respiration is repressed). This indicates that the hxk2 deletion strain has a strongly enhanced oxidative capacity when grown on a mixture of glucose and ethanol.

The aromatic character of the region 66ywYWW7° ending at 66Y fused to residues 67SCSQAC72 coming from a of the ll-kDa subunit VIII of ubiquinol-cytochrome c oxidostop-oligonucleotide [5]. . This mutant the present study, three other site-directed mutants (66SAYAA7°, showed a reduced enzymic activity and alterations in the bind-66SASAWT° and ~SWYWW 7°) were constructed and analysed, ing of the Qout inhibitor myxothiazol, implying that this reThe data indicate that the presence of one aromatic residue is enough for a substantial level of assembly and that its position gion of the yeast 11-kDa protein contributes to the Qout bindmodulates the level of both assembly and electron transfer ing site. Additionally, the region 66ywYw~V7° in the S. activity. The results also confirm the relevance of this region of cerevisiae subunit VIII was replaced by non-aromatic residues subunit VIII for the formation of the Qout reaction domain, as (66SASAA7°) by site-directed mutagenesis, resulting in the reported by Hemrika et al. [(1993) Eur. J. Biochem. 215, 601-nearly complete absence of assembled complex III [4]. Based 609]. It is further shown that the lowered specific activity of the on the occurrence of a spontaneous second mutation, the mutant described by these authors is not due to the introduction 66FASAAT° mutant, which had a 60-70% recovery of the of a cysteine in the sequence of subunit VIII.wild-type activity, it was suggested that the aromatic nature of residue 66 was important for assembly of a functional en-