Genes encoding homologues of known amino-acid permeases were deleted in a strain also deficient in the general amino-acid permease. The uptake capacity of the mutants was investigated for several L-alpha-amino acids. Deletion of a gene denoted DIP5 results in the loss of L-aspartate and L-glutamate uptake. The dip5 mutation caused a several hundred-fold reduction of uptake of the two amino acids, both in cells grown on proline as a nitrogen source and in cells grown on ammonium. DIP5-dependent uptake of L-aspartate and L-glutamate was somewhat lower in ammonium-grown cells than in proline-grown cells. Transcriptional regulation is at least partially responsible for this difference, as shown by assaying the DIP5 promoter fused to lacZ. This suggests that the promoter is subject to nitrogen catabolite repression. Transport of a few other amino acids was moderately affected by dip5 but was not competed by L-aspartate in the DIP5 parental strain; transport of these amino acids is therefore unlikely to be mediated by Dip5p. Our results suggest that DIP5 encodes an amino-acid permease with a high transport capacity and a high affinity for L-glutamate and L-aspartate, with a Kt of about 50 microM for both.