Protein-dependent group II intron splicing provides a forum
for exploring the roles of proteins in facilitating RNA-catalyzed
reactions. The maize nuclear gene crs1 is required
for the splicing of the group II intron in the chloroplast
atpF gene. Here we report the molecular cloning of
the crs1 gene and an initial biochemical characterization
of its gene product. Several observations support the notion
that CRS1 is a bona fide group II intron splicing factor. First,
CRS1 is found in a ribonucleoprotein complex in the chloroplast,
and cofractionation data provide evidence that this complex
includes 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 function found in
both the eubacteria and archaea. crs1 is the founding
member of a gene family in plants that was derived by duplication
and divergence of this primitive gene. In addition to its
previously established role in atpF intron splicing,
new genetic data implicate crs1 in chloroplast
translation. The chloroplast splicing and translation functions
of crs1 may be mediated by the distinct protein products
of two crs1 mRNA forms that result from alternative
splicing of the crs1 pre-mRNA.