The gene-number paradox.

Abstract

Saccharomyces cerevisiae chromosome I has provided a vivid example of the "gene-number paradox." Although molecular studies have suggested that there are $>$100 transcribed regions on the chromosome, classical genetic studies have identified only about 15 genes, including intensive searches for Ts$\sp-$-lethal mutants. To help elucidate the reasons for the disparity, a detailed molecular analysis of a 52 kb segment of the left arm of the chromosome has been undertaken. This segment contains the four known genes CDC24, WHI1, CYC3, and PYK1 plus at least 13 transcribed regions of unknown function. The 17 identified transcripts have a total length of $\sim$39 kb, suggesting that $\geq$75% of the DNA in this region is transcribed. Of the transcribed regions of unknown function, six are essential for viability on rich medium and five appear to be nonessential, as judged by the lethality or nonlethality of deletions constructed using integrative transformation methods. It is not known whether the two remaining genes are essential or not. No obvious phenotypes were associated with the deletions in the apparently nonessential genes. However, two of these genes may have homologues elsewhere in the genome, as judged from the appearance of additional bands when DNA-DNA blot hybridizations were performed at reduced stringency. Taken together, these results provide further evidence that the limitations of classical genetic studies of chromosome I cannot be explained solely by a lack of genes, or even a lack of essential genes, on the chromosome. These results emphasize two new questions. Why are there so many nonessential genes? What roles do these genes play? In addition, it seems clear that there are many essential genes on chromosome I that have not been identified in prior classical genetic studies, including the intensive analyses of Ts$\sp-$-lethal mutants. What are the possible explanations for this situation?