CRS1 is a novel group II intron splicing factor that was derived from a domain of ancient origin

Protein-dependent group If intron splicing provides a forum for exploring t he roles of proteins in facilitating RNA-catalyzed reactions. The maize nuc lear gene crs1 is required for the splicing of the group II intron in the c hloroplast atpF gene. Here we report the molecular cloning of the crs1 gene and an initial biochemical characterization of its gene product. Several o bservations support the notion that CRS1 is a bona fide group If intron spl icing factor. First, CRS1 is found in a ribonucleoprotein complex in the ch loroplast, and cofractionation data provide evidence that this complex incl udes atpF intron RNA. Second, CRS1 is highly basic and includes a repeated domain with features suggestive of a novel RNA-binding domain. This domain is related to a conserved free-standing open reading frame of unknown funct ion found in both the eubacteria and archaea. crs1 is the founding member o f a gene family in plants that was derived by duplication and divergence of this primitive gene. In addition to its previously established role in atp F intron splicing, new genetic data implicate crs1 in chloroplast translati on. The chloroplast splicing and translation functions of crs1 may be media ted by the distinct protein products of two crs1 mRNA forms that result fro m alternative splicing of the crs1 pre-mRNA.