Control-mechanisms acting at the transcriptional and post-transcriptional levels are involved in the synthesis of the arginine pathway carbamoylphosphate synthase of yeast.

In Saccharomyces cerevisiae, the synthesis of the arginine pathway enzyme carbamoylphosphate synthase (CPSase A) is subject to two control mechanisms. One mechanism, the general control of amino acid biosynthesis, influences the expression of both CPA1 and CPA2 genes, the structural genes for the two subunits of the enzyme. The second mechanism, the specific control of arginine biosynthesis, only affects the expression of CPA1. To study these mechanisms in more detail, we have cloned the CPA1 and CPA2 genes and used their DNA to measure the CPA1 and CPA2 mRNA content of cells grown under various conditions. A close coordination was observed in the variation of the levels of CPA1 and CPA2 mRNAs and polypeptide products under conditions where the general control of amino acid biosynthesis operates. In contrast, little correlation was found between the levels of CPA1 mRNA and the corresponding protein for conditions affecting repression by arginine: the total amplitude of variation was 6-fold higher for the CPA1 protein than for the CPA1 messenger transcript. Such findings are consistent with the conclusion that the general control operates at the transcriptional level and that the specific arginine control acts primarily at a post-transcriptional level.