Evolution of Yeast Noncoding RNAs Reveals an Alternative Mechanism for Widespread Intron Loss

Abstract

Kicking Out Introns Many genes in eukaryotes contain introns that must be removed from the messenger RNA for proper gene function. Humans have on average eight introns per gene, whereas more than 90% of the genes in the yeast species Saccharomyces cerevisiae and Candida albicans have none at all. To understand how introns can be lost from genes, Mitrovich et al. (p. 838 ) compared non–protein-coding genes among the yeasts and found that genes for small nucleolar RNAs (snoRNAs) in C. albicans are intronic. By contrast in S. cerevisiae , snoRNAs are processed from unmodified RNA, suggesting a massive loss of snoRNAs—associated introns in the common ancestor of the Saccharomyces species. The introns seem to have been lost through splice-site degeneration, and associated compaction of linked exons resulted in nested splicing of some snoRNAs.