A COMPARATIVE-STUDY ON THE TRANSPORT OF L(-)MALIC ACID AND OTHER SHORT-CHAIN CARBOXYLIC-ACIDS IN THE YEAST CANDIDA-UTILIS - EVIDENCE FOR A GENERAL ORGANIC-ACID PERMEASE

Cells of the yeast Candida utilis grown in medium with short-chain mon o-, di- or tricarboxylic acids transported L(-)malic acid by two trans port systems at pH 3.0. Results indicate that probably a proton sympor t for the ionized form of the acid and a facilitated diffusion for the undissociated form were present. Dicarboxylic acids such as succinic, fumaric, oxaloacetic and a-ketoglutaric acids were competitive inhibi tors of the malic acid for the high-affinity system, suggesting that t hese acids used the same transport system. In turn, competitive inhibi tion uptake studies of labelled carboxylic acid in the low-affinity ra nge indicated that this system was non-specific and able to accept not only carboxylic (mono-, di- or tri-) acids but also some amino acids. Additionally, under the same growth conditions, C. utilis produced tw o mediated transport systems for lactic acid: a proton symport for the anionic form which appeared to be a common monocarboxylate carrier an d a facilitated diffusion system for the undissociated acid displaying a substrate specificity similar to that observed for the low-affinity dicarboxylic acid transport. The mediated carboxylic acid transport s ystems were inducible and subjected to repression by glucose. In gluco se-grown cells the undissociated dicarboxylic acids entered the cells slowly by simple diffusion. Repressed glucose-grown cells were only ab le to produce both transport systems if an inducer, at low concentrati on (0.5%, w/v), was present during starvation in buffer. This process was inhibited by the presence of cycloheximide indicating that inducti on requires de novo protein synthesis. If a higher acid concentration was used, only the low-affinity transport system was detectable, showi ng that the high-affinity system was also repressed by high concentrat ions of the inducer.