Résumé : In Saccharomyces cerevisiae, the pathway of 4-aminobutyric acid catabolism, for use as a nitrogen source, involves a specific permease (encoded by the UGA4 gene) and two enzymes (encoded by the UGA1 and UGA2 genes, respectively). The synthesis of these proteins is induced by 4-aminobutyrate. It also requires the product of the UGA3 gene. Here, we describe four additional regulatory mutations which provide evidence for the existence of both positive and negative regulatory elements which control the final expression of the UGA4 gene. Some of them simultaneously control the expression of the UGA1 and UGA2 genes. Three classes of mutant with a constitutive 4-aminobutyrate-specific permease have been isolated. (a) Recessive mutations in the UGA43 gene suggest that the product of the UGA43 gene behaves like a trans-acting negative regulator of UGA4 gene expression. (b) The semi-dominant mutation (uga11), closely linked to the UGA4 gene, might affect the receptor of the UGA43 gene product. In these two classes of mutant, only the permease is constitutive. (3) The uga81 mutation, closely linked to the UGA3 gene, makes the whole UGA regulon constitutive. On the other hand, recessive mutations at the UGA35 gene locus lead to non-inducibility of the UGA regulon. Hence the UGA35 gene product behaves like a second trans-acting positive regulator in addition to UGA3.