Dramatically reduced spliceosome in Cyanidioschyzon merolae

The spliceosome—the molecular particle responsible for removing interrupting sequences from eukaryotic messenger RNA—is one of the most complex cellular machines. Consisting of five snRNAs and over 200 proteins in humans, its numerous changes in composition and shape during splicing have made it difficult to study. We have characterized an algal spliceosome that is much smaller, with only 43 identifiable core proteins, the majority of which are essential for viability in other organisms. We propose that this highly reduced spliceosome has retained only the most critical splicing factors. Cyanidioschyzon merolae therefore provides a powerful system to examine the spliceosome’s catalytic core, enabling future advances in understanding the splicing mechanism and spliceosomal organization that are challenging in more complex systems.