The high frequency of alternative splicing among the serine/arginine-rich (SR) family of proteins in plants has been linked to important roles in gene regulation during development and in response to environmental stress. In this article, we have searched and manually annotated all the SR proteins in the genomes of maize and sorghum. The experimental validation of gene structure by reverse transcription-polymerase chain reaction (RT-PCR) analysis revealed, with few exceptions, that SR genes produced multiple isoforms of transcripts by alternative splicing. Despite sharing high structural similarity and conserved positions of the introns, the profile of alternative splicing diverged significantly between maize and sorghum for the vast majority of SR genes. These include many transcript isoforms discovered by RT-PCR and not represented in extant expressed sequence tag (EST) collection. However, we report the occurrence of various maize and sorghum SR mRNA isoforms that display evolutionary conservation of splicing events with their homologous SR genes in Arabidopsis and moss. Our data also indicate an important role of both 5' and 3' untranslated regions in the regulation of SR gene expression. These observations have potentially important implications for the processes of evolution and adaptation of plants to land.
Keywords: SR proteins; alternative splicing; evolutionary conservation.