Pseudogenes in yeast?

In addition to the ensemble of intact, expressed genes the chromosomes of many vertebrates are cluttered with degenerate copies called pseudogenes (Vanin, Ann. Rev. Genet. 79,253-272,1985). One type of pseudogene, the “processed pseudogene,” is usually found at a different chromosomal location from the expressed gene and often contains mutations in the coding region. Processed pseudogenes lack promoter sequences and introns and contain a 3’poly(A) stretch that is missing from the same portion of the expressed gene. These features of processed pseudogenes have led to speculation that they weie generated from reverse transcripts of cellular RNAs. Surprisingly, the yeast Saccharomyces cerevisiae has few, if any, processed pseudogenes despite the fact that yeast cells have both a reverse transcriptase and several genes with introns. The yeast reverse transcriptase is encoded by the Ty transposon, a repetitive DNA element whose structure is similar to that of retroviral proviruses found in vertebrates (Garfinkel et al., Cell 42, 507-517, 1985). The level of Ty reverse transcriptase is normally low, but can be much higher in a strain containing a genetically engineered Ty element. If this enzyme were able to reverse transcribe cellular mRNAs, then one might expect the yeast genome to be peppered with pseudogenes, like the vertebrate genome. Yet no gene in yeast has the features of a typical processed pseudogene. A second striking feature of the Saccharomyces genome is the paucity of genes with introns. Of the several hundred genes that have been sequenced, only seventeen contain introns. The infrequent occurrence of introns in Saccharomyces is not some peculiar quirk of fungi since the distribution of introns within the genes of Neurospora and Schizosaccharomyces pombe is not different from that found in plants and vertebrates. The existence of pseudogenes in vertebrates and their apparent absence from Saccharomyces could be a consequence of the nature of the recombination process that leads to cDNA integration. In mammalian cells, exogenous DNA introduced by transfection usually integrates by nonhomologous recombination. If an endogenous cDNA were integrated by the same process, the resultant gene copy would have all the features of a processed pseudogene (new chromosomal location, poly(A), no intron, no promoter). By contrast, exogenous DNA introduced into yeast always integrates by homologous recombination. This recombination system is extremely active, as shown by the fact that the yeast genetic map is roughly the same size as that of humans (3300 centiMorgans) even though a yeast cell has only about 1% as much DNA as a human cell. If enMinireview