Arabidopsis TITAN LIKE is required for U12-type intron splicing, especially for AT–AC subtypes

Many eukaryotes have two types of spliceosome, i.e., the U2-dependent and U12-dependent spliceosomes. The terminal dinucleotides of almost all introns spliced by the U2-dependent spliceosome are GT–AG, whereas the most of the rest involved the U12-dependent spliceosome ending in AT–AC. How the U12-dependent spliceosome splices two types of introns with different ends also remains poorly understood. For example, it has been reported that human CENATAC is a subunit of U12-dependent spliceosome and is—uniquely—required for the splicing of AT–AC introns. The Arabidopsis genome contains one gene called TITAN LIKE ( TTL ), a homolog of CENATAC , but the function this plays in splicing is unknown. In this paper we created a weak TTL mutant named ttl-142 , and investigated the function of TTL during splicing. We found that ttl-142 had a 14-residue deletion, and that homozygous mutants showed morphological abnormalities. Most U12-dependent introns were less spliced in ttl-142 than in the wild type, and the splicing of AT–AC introns was particularly suppressed. Splicing suppression in ttl-142 was more extensive than in a DROL1 mutant, which contains a mutation in a gene that is specifically required for AT–AC intron splicing. In contrast, fewer genes showed changes in expression levels in ttl-142 than in drol1 , and most DEGs in ttl-142 and drol1 were different. These results suggest that the phenotypes of ttl-142 and drol1 mutants may correspond to the impairment of specific spliceosomal functions.