atSRp30, one of two SF2/ASF-like proteins from Arabidopsis thaliana, regulates splicing of specific plant genes

SR proteins are nuclear phosphoproteins with a characteristic Ser/Arg-rich domain and one or two RNA recognition motifs. They are highly conserved in animals and plants and play important roles in spliceosome assembly and alt ernative splicing regulation. We have now isolated and partially sequenced a plant protein, which crossreacts with antibodies to human SR proteins. Th e sequence of the corresponding cDNA and genomic clones from Arabidopsis re vealed a protein, atSRp3a, with strong similarity to the human SR protein S F2/ASF and to atSRp34/SR1, a previously identified SR protein, indicating t hat plants possess two SP2/ASF-like proteins, atSRp30 expresses alternative ly spliced mRNA isoforms that are expressed differentially in various organ s and during development. Overexpression of atSRp30 via a strong constituti ve promoter resulted in changes in alternative splicing of several endogeno us plant genes, including atSRp30 itself. Interestingly, atSRp30 overexpres sion resulted in a pronounced down-regulation of endogenous mRNA encoding f ull-length atSRp34/SR1 protein. Transgenic plants overexpressing atSRp30 sh owed morphological and developmental changes affecting mostly developmental phase transitions. atSRp30- and atSRp34/SR1-promoter-GUS constructs exhibi ted complementary expression patterns during early seedling development and root formation, with overlapping expression in floral tissues. The results of the structural and expression analyses of both genes suggest that atSRp 34/SR1 acts as a general splicing factor, whereas atSRp30 functions as a sp ecific splicing modulator.