Transcription of the ARG4 gene of Saccharomyces cerevisiae is regulated by general control of amino acid biosynthesis but not by a specific regulatory mechanism. Three deletion mutants (delta I, delta II, delta III) successively removing DNA sequences upstream from the coding sequence have been phenotypically analyzed after insertion into a single copy plasmid. As expected, delta I, which lacks the sequences upstream to -155, including the two putative upstream activation sequences (UAS), was unable to derepress argininosuccinate lyase biosynthesis under conditions of amino acid starvation. In delta II (deleted up to -126) the enzyme activity was very low and cells harbouring this allele were arginine dependent. These drastic phenotypic changes can be attributed to the loss of 12 out of 14 dA residues from positions -124 to -137. This poly (dAdT) sequence most likely serves as an upstream promoter element for constitutive expression of ARG4. The delta III deletion removes all 5' sequences including the putative TATA box. This inactive allele has been successfully used for selecting yeast promoters of unknown origin.