DROL1/DIB1 determines U5 snRNP specificity for intron terminal dinucleotide in Arabidopsis

Most introns contain GT–AG terminal dinucleotides; although some eukaryotes have introns with AT–AC termini whose splicing is impaired in the Arabidopsis defective repression of OLE3::LUC 1 (drol1) mutant. We identified seven drol1 suppressors across four loci, all encoding subunits of the U5 snRNP. Although AT–AC splicing was partially restored in these suppressors, their phenotypes were almost completely rescued. Artificial introns with either GT–AG or AT–AC termini showed a splicing preference for GT–AG termini in drol1 and its suppressors. These results suggest that AT–AC introns are spliced by a GT–AG specific spliceosome in the suppressors, with DROL1 influencing U5 snRNP specificity at the 5′ splice site. We propose that mRNAs retaining unspliced AT–AC introns impair translation and produce nuclear signals that contribute to the complex phenotypes seen in drol1.