Insights into the mechanism of pre-mRNA splicing of tiny introns from the genome of a giant ciliate Stentor coeruleus

Abstract

Stentor coeruleus is a ciliate known for its regenerative ability. Recent genome sequencing reveals that its spliceosomal introns are exceptionally small. We wondered whether the multi-megadalton spliceosome has any unique characteristics for removal of the tiny introns. First, we analyzed intron features and identified spliceosomal RNA/protein components. We found that all snRNAs are present, whereas many proteins are conserved but slightly reduced in size. Some regulators such as Serine/Arginine-rich proteins are noticeably undetected. Interestingly, while most parts of spliceosomal proteins, including Prp8’s positively-charged catalytic cavity, are conserved, regions of branching factors projecting to the active site are not. We conjecture that steric-clash avoidance between spliceosomal proteins and a sharply-looped lariat might occur and splicing regulation may differ from other species.