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Genes of the baker's yeast Saccharomyces cerevisiae are densely clustered on 16 linear chromosomes. Here, I characterize a 1.8 kb region of chromosome XV containing the entire structural gene for the histidine biosynthetic enzyme imidazoleglycerolphosphate (IGP) dehydratase (his3) as well as the promoter sequences and 5'-proximal mRNA coding regions for the adjacent genes. The his3 gene encodes several mRNA species averaging 820 bases in length, all of which contain an open reading frame of 219 codons. The location of this open reading frame coincides with the his3 gene as defined by functional criteria, suggesting that the primary translation product of yeast IGP dehydratase has a molecular weight of 23,850. Phenotypic analysis of mutations constructed in vitro indicate that one of the adjacent genes (pet56) is required for mitochondrial function, whereas the other gene (ded1) is essential for cell viability. The pet56 and his3 genes are transcribed divergently from initiation sites that are separated by only 192 bp. Transcription of the ded1 gene is initiated only 130 bp beyond the 3'-end of the his3 mRNA coding region. These results suggest that these unrelated genes are located extremely close together and that the spacer regions between them consist largely of promoter and terminator sequences.
Kevin Struhl (Tue,) studied this question.