A SEGMENT OF MESSENGER-RNA ENCODING THE LEADER PEPTIDE OF THE CPA1 GENE CONFERS REPRESSION BY ARGININE ON A HETEROLOGOUS YEAST GENE TRANSCRIPT

The expression of the yeast gene CPA1, which encodes the small subunit of the arginine pathway carbamoylphosphate synthetase, is repressed b y arginine at a translational level. CPA1 mRNA contains a 250-nucleoti de-long leader which includes a 25-codon upstream open reading frame ( uORF). Oligonucleotide site-directed mutagenesis of this uORF as well as sequencing of constitutive cis-dominant mutations has suggested tha t the leader peptide product of the CPA1 uORF is an essential negative element for repression of the CPA1 gene by arginine. In this work, a series of deletions affecting the regions 5' and 3' to the uORF in the leader sequence was constructed. The arginine-dependent repression of CPA1 was little affected in these constructions, indicating that thes e regions are not essential for the regulatory response. This conclusi on was further supported by the finding that inserting the mRNA segmen t encoding the leader peptide sequence of CPA1 in the leader sequence of another gene, namely, GCN4, places this gene under arginine repress ion. Similarly, the behavior of fusions of the leader sequence of CPA1 with those of ARG4 or GAL10 confirmed that the regions of this leader located upstream and downstream from the uORF are dispensable for the regulation by arginine. Finally, a set of substitution mutations whic h modify the uORF nucleotide sequence while leaving unchanged the corr esponding amino acid sequence was constructed. The mutations did not a ffect the repression of CPA1 by arginine. The data presented in this p aper consequently agree with the conclusion that the leader peptide it self is the main element required for the translational repression of CPA1.