Origins of transcripts of the yeast mitochondrial var 1 gene

Transcription of the yeast mitochondrial var 1 gene encoding the mitochondrial small ribosomal subunit protein, var 1, has been examined in wild type and petite strains by Northern hybridization, S 1 nuclease protection experiments, and analysis of primary transcripts by in vitro labeling of mitochondrial RNAs with [alpha-32P]GTP and guanylyl transferase. We have identified a transcription unit in which a polycistronic precursor gives rise to the mRNAs for ATPase subunit 9 (the product of the oli 1 gene), var 1, and tRNASerUCN. Potential cleavage sites have been identified within this precursor that separates oli 1, tRNASerUCN, and var 1 transcripts. Transcriptional initiation of the polycistronic precursor occurs from one of two nearby copies of the sequence, 5'-ATATAAGTA-3', located approximately 550 and 630 base pairs 5' to the oli 1 gene. A closely related sequence, 5'-TTATAAGTA-3', is found within the var 1 gene and may serve as a transcriptional initiation site for one of a pair of primary transcripts detected in a petite retaining the var 1[40.0] allele. The other primary transcript in this strain initiates upstream in a region of the var 1 gene containing no recognizable initiation sites.