Sugars are excellent carbon sources for all yeasts. Since a vast amount of information is available on the components of the pathways of sugar utilization in Saccharomyces cerevisiae it has been tacitly assumed that other yeasts use sugars in the same way. However, although the pathways of sugar utilization follow the same theme in all yeasts, important biochemical and genetic variations on it exist. Basically, in most non-conventional yeasts, in contrast to S. cerevisiae, respiration in the presence of oxygen is prominent for the use of sugars. This review provides comparative information on the different steps of the fundamental pathways of sugar utilization in non-conventional yeasts: glycolysis, fermentation, tricarboxylic acid cycle, pentose phosphate pathway and respiration. We consider also gluconeogenesis and, briefly, catabolite repression. We have centered our attention in the genera Kluyveromyces, Candida, Pichia, Yarrowia and Schizosaccharomyces, although occasional reference to other genera is made. The review shows that basic knowledge is missing on many components of these pathways and also that studies on regulation of critical steps are scarce. Information on these points would be important to generate genetically engineered yeast strains for certain industrial uses. (C) 2000 Federation of European Microbiological Societies.

Work in the laboratory of the authors was supported by Grant PB97-1213-CO2-01 from the Spanish CICYT and by Project BIO4-CT95-0132 `From gene to products in yeasts : a quantitative approach' from the European Union (DGXII Framework IV, Program of Cell Factories). C.R. benefitted from a Fellowship from the Fundación Ramón Areces (Madrid, Spain). The stay of T.P. in Delft is supported by the Marie Curie Biotechnology Program from the European Union (Grant ERB-4001GT980575/Project IO4-CT98-5096).

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