In vivo transcription of a eukaryotic regulatory gene.

The PPR1 gene encodes the positive regulator of the URA1 and URA3 genes in yeast. Its transcription product is a 2.9-kb polyadenylated RNA with an extremely short half-life of 1 min. The induced or non-induced cell contains approximately 0.1 molecules of PPR1 RNA, a constitutive level which is not altered by changing the number of structural genes to be regulated. The DNA sequence of a 399-bp AccI-Bg/II fragment including 180 nucleotides of the 5'-flanking region of the gene PPR1 has been determined. By S1 mapping we present evidence that the 5' non-coding region of the PPR1 mRNA is heterogeneous in length. The 5' termini of the different RNAs map 20, 36, 45 and 50 nucleotides upstream from the translation start codon. The sequence indicates that the only open translation phase begins with an AUG codon that is preceded by two out-of-frame AUG triplets. This particular structure of the 5'-terminal sequence of the transcripts of PPR1 is discussed in relation to both their stability and translational efficiency.