Identifying RNA splicing factors using genes in .

Intraflagellar transport moves proteins in and out of flagella/cilia and it is essential for the assembly of these organelles. Using whole-genome sequencing, we identified splice site mutations in two genes, () and (), which lead to flagellar assembly defects in the unicellular green alga The splicing defects in these mutants are partially corrected by mutations in two conserved spliceosome proteins, DGR14 and FRA10. We identified a deletion mutant, which suppresses the 3' splice site mutation in , and a frameshift mutant of , which suppresses the 5' splice site mutation in Surprisingly, we found and mutations suppress both splice site mutations. We suggest these two proteins are involved in facilitating splice site recognition/interaction; in their absence some splice site mutations are tolerated. Nonsense mutations in , which is involved in nonsense-mediated decay, lead to accumulation of aberrant transcripts and partial restoration of flagellar assembly in the mutants. The high density of introns and the conservation of noncore splicing factors, together with the ease of scoring the mutant phenotype, make an attractive organism to identify new proteins involved in splicing through suppressor screening.