Dramatically reduced spliceosome inCyanidioschyzon merolae

• Published in: Proceedings of the National Academy of Sciences - PNAS 2015-03, Vol.112 (11), p.E1191-E1200
• Main Authors: Stark, Martha R., Dunn, Elizabeth A., Dunn, William S. C., Grisdale, Cameron J., Daniele, Anthony R., Halstead, Matthew R. G., Fast, Naomi M., Rader, Stephen D.
• Format: Article
• Language: English
• ISSN: 0027-8424; 1091-6490

The human spliceosome is a large ribonucleoprotein complex that catalyzes pre-mRNA splicing. It consists of five snRNAs and more than 200 proteins. Because of this complexity, much work has focused on theSaccharomyces cerevisiaespliceosome, viewed as a highly simplified system with fewer than half as many splicing factors as humans. Nevertheless, it has been difficult to ascribe a mechanistic function to individual splicing factors or even to discern which are critical for catalyzing the splicing reaction. We have identified and characterized the splicing machinery from the red algaCyanidioschyzon merolae, which has been reported to harbor only 26 intron-containing genes. The U2, U4, U5, and U6 snRNAs contain expected conserved sequences and have the ability to adopt secondary structures and form intermolecular base-pairing interactions, as in other organisms.C. merolaehas a highly reduced set of 43 identifiable core splicing proteins, compared with ∼90 in budding yeast and ∼140 in humans. Strikingly, we have been unable to find a U1 snRNA candidate or any predicted U1-associated proteins, suggesting that splicing inC. merolaemay occur without the U1 small nuclear ribonucleoprotein particle. In addition, based on mapping the identified proteins onto the known splicing cycle, we propose that there is far less compositional variability during splicing inC. merolaethan in other organisms. The observed reduction in splicing factors is consistent with the elimination of spliceosomal components that play a peripheral or modulatory role in splicing, presumably retaining those with a more central role in organization and catalysis.