The Saccharomyces cerevisiae ARG1 gene coding for argininosuccinate synthetase has been isolated and the nucleotide sequence of both its control region and of its amino terminal end coding region determined. The startpoint of transcription was established by S1-mapping and reverse transcriptase procedures. Northern blot hybridizations showed that whereas arginine-specific repression reduced the enzyme activity fivefold, it did not reduce the steady state level of the corresponding messenger in proportion; by analogy with the coregulated ARG3 gene, this result suggests a post-transcriptional regulatory mechanism. In contrast, proportionally between enzyme activity and mRNA content was observed under conditions where general amino acid control (known to be transcriptional) was operating. Comparing the 5' untranscribed domains of ARG1 and ARG3 revealed a first region of homology between the TATA box and the transcription startpoint. In this region a 10 bp (ARG3) or 11 bp (ARG1) central box is flanked by two segments which, by mutation, have been shown to be part of the ARG operator (Crabeel et al. 1985). The repressor is assumed to bind at this primary target site prior to establishing contacts with the proximal part of the nascent mRNA molecule (Crabeel et al. 1985). By in vitro directed deletion mutagenesis we show that the central conserved box of ARG3 is not essential for arginine-specific repression to occur. Another region of homology was found in the leader part of the messenger RNA; deletion of this region causes a small reduction in ARG3 expression but also does not alter regulation. Neither of these two regions are thus part of the primary repressor target site. In addition, in terms of post-transcriptional regulation, the latter result indicates that no sequence specificity is required in the RNA recognition step.