Identification and Validation of the U2, U4, U5, and U6 Spliceosomal snRNAs in Cyanidioschyzon merolae.

Pre-mRNA splicing is an essential step in eukaryotic gene expression, and introns have been found in nearly all eukaryotic genomes sequenced to date. The red alga Cyanidioschyzon merolae is found in acidic thermal springs, and its recently sequenced genome revealed a surprising paucity of intron-containing genes, raising the question of whether the normal complement of splicing machinery is maintained to splice so few introns. To address this I searched for snRNAs computationally, successfully identifying C. merolae homologues for four of the five snRNAs. I experimentally confirmed their expression, found that their structural elements are similar to those known from other organisms, and demonstrated that U4 and U6 base pair to each other, as expected. My data support the proposed switch in U6-5's splice site base pairing between the two catalytic steps, as well as a recent model for free U6. --P. ii.