Mutants deficient in amino acid transport (aau), and unable to utilize L-glutamate as a sole carbon and nitrogen source, have been isolated. There are four unlinked genes involved: aauA, aauB, aauC and aauD. The transport levels of certain amino acids, and the growth characteristics on certain nitrogen and carbon sources and toxic amino acid analogues, indicate that: aauAI has defective transport of acidic amino acids; aauBI has defective transport of acidic and neutral amino acids; aauCI and aauDI have defective transport of acid, neutral and aromatic amino acids. aauAI and aauBI are recessive for all three characteristics in the heterozygous diploid; aauCI and aauDI are dominant.