The Serine/Arginine-Rich Protein Family in Rice Plays Important Roles in Constitutive and Alternative Splicing of Pre-mRNA W

Ser/Arg-rich (SR) proteins play important roles in the constitutive and alternative splicing of pre-mRNA. We isolated 20 rice (Oryza sativa) genes encoding SR proteins, of which six contain plant-specific characteristics. To determine whether SR proteinsmodulate splicing efficiency and alternative splicing of pre-mRNA in rice,we used transient assays in rice protoplasts by cotransformation of SR protein genes with the rice Waxyb (Wxb)-b-glucuronidase fusion gene. The results showed that plant-specific RSp29 and RSZp23, an SR protein homologous to human 9G8, enhanced splicing and altered the alternative 59 splice sites ofWxb intron 1. The resulting splicingpatternwasunique toeachSRprotein; RSp29 stimulated splicing at thedistal site, and RSZp23 enhanced splicing at the proximal site. Results of domain-swapping experiments between plant-specific RSp29 andSCL26,which is a homologof humanSC35, showed the importance of RNA recognitionmotif 1 and theArg/Ser-rich (RS) domain for the enhancement of splicing efficiencies. Overexpression of plant-specific RSZ36 and SRp33b, a homolog of human ASF/SF2, in transgenic rice changed the alternative splicing patterns of their own pre-mRNAs and those of other SR proteins. These results show that SR proteins play important roles in constitutive and alternative splicing of rice pre-mRNA.