Conservation of mRNA quality control factor Ski7 and its diversification through changes in alternative splicing and gene duplication.

Eukaryotes maintain fidelity of gene expression by preferential degradation of aberrant mRNAs that arise by errors in RNA processing reactions. In Saccharomyces cerevisiae, Ski7 plays an important role in this mRNA quality control by mediating mRNA degradation by the RNA exosome. Ski7 was initially thought to be restricted to Saccharomyces cerevisiae and close relatives because the SKI7 gene and its paralog HBS1 arose by whole genome duplication (WGD) in a recent ancestor. We have recently shown that the preduplication gene was alternatively spliced and that Ski7 function predates WGD. Here, we use transcriptome analysis of diverse eukaryotes to show that diverse eukaryotes use alternative splicing of SKI7/HBS1 to encode two proteins. Although alternative splicing affects the same intrinsically disordered region of the protein, the pattern of splice site usage varies. This alternative splicing event arose in an early eukaryote that is a common ancestor of plants, animals, and fungi. Remarkably, through changes in alternative splicing and gene duplication, the Ski7 protein has diversified such that different species express one of four distinct Ski7-like proteins. We also show experimentally that the Saccharomyces cerevisiae SKI7 gene has undergone multiple changes that are incompatible with the Hbs1 function and may also have undergone additional changes to optimize mRNA quality control. The combination of transcriptome analysis in diverse eukaryotes and genetic analysis in yeast clarifies the mechanism by which a Ski7-like protein is expressed across eukaryotes and provides a unique view of changes in alternative splicing patterns of one gene over long evolutionary time.