Many splicing factors in vertebrate nuclei belong to a class of evolut
ionarily conserved proteins containing arginine/serine (RS) or serine/
arginine (SR) domains, Previously, we demonstrated the existence of SR
splicing factors in plants. In this article, we report on a novel mem
ber of this splicing factor family from Arabidopsis designated atRSp31
. It has one N-terminal RNA recognition motif and a C-terminal RS doma
in highly enriched in arginines. The RNA recognition motif shows signi
ficant homology to all animal SR proteins identified to date, but the
intermediate region does not show any homology to any other known prot
ein. Subsequently, we characterized two cDNAs from Arabidopsis that ar
e highly homologous to atRSp31 (designated atRSp35 and atRSp41). Their
deduced amino acid sequences indicate that these proteins constitute
a new family of RS domain splicing factors. Purified recombinant atRSp
31 is able to restore splicing in SR protein-deficient human S100 extr
acts. This indicates that atRSp31 is a true plant splicing factor and
plays a crucial role in splicing, similar to that of other RS splicing
factors. All of the three genes are differentially expressed in a tis
sue-specific manner, The isolation of this new plant splicing factor f
amily enlarges the essential group of RS domain splicing factors. Furt
hermore, because no animal equivalent to this protein family has been
identified to date, our results suggest that these proteins play key r
oles in constitutive and alternative splicing in plants.