Increased copy number of the 5' end of the SPS2 gene inhibits sporulation of Saccharomyces cerevisiae

We found that the introduction into a yeast cell of a high-copy-number plasmid containing the 5' end of the SPS2 gene, a sporulation-specific gene of Saccharomyces cerevisiae, led to a reduction in the efficiency of spore formation. The plasmid pAP290, which contains the sequence from -138 to +152 of the SPS2 gene, caused a fivefold reduction in spore formation; the presence of the plasmid had no effect on transcription of the chromosomal SPS2 gene. A plasmid containing only the sequence upstream of the TATA box of the SPS2 gene (-350 to -68) was unable to inhibit the completion of sporulation, whereas the downstream sequence, from -70 to +404, although unable by itself to inhibit sporulation, could do so when provided with an upstream fragment containing the CYC1 upstream activation sequence. Deletion of 22 base pairs from pAP290, which introduced a frameshift after codon 17 of the SPS2 gene and reduced the open reading frame to 26 amino acids, generated a plasmid (pAP290 delta Pst) which could no longer inhibit sporulation. The SPS2 inserts of pAP290 and pAP290 delta Pst were found to direct equivalent levels of sporulation-specific transcription. We conclude from these results that the presence of both the SPS2 promoter (or a substitute promoter) and the initial coding sequence of the SPS2 gene is required in the high-copy-number plasmid to generate the asporogenous phenotype. We speculate that the accumulation of a protein containing the amino-terminal portion of the SPS2 gene product, synthesized from the transcripts of the truncated plasmid-borne copies of the SPS2 gene, prevents ascus formation.