2008, Alternative splicing at NAGNAG acceptor sites shares common No. 3] K. Iida et al. 163 properties in land plants and mammals

In recent years, several papers have reported that a special type of alternative splicing (AS) event occurs at the tandem 3# splice site, termed the ''NAGNAG acceptor.'' This type of AS event (termed AS-NAGNAG) is well studied in both human and mouse. To illustrate the significance of AS-NAGNAG events, we focused on their occurrence in Arabidopsis thaliana and Oryza sativa (rice). Our study is the first genome-wide approach examining AS-NAGNAG events in land plants. Based on transcripts and genomic sequences, we found 321 and 372 AS-NAGNAG events in Arabidopsis and rice, respectively. These events were significantly enriched in genes encoding DNA-binding proteins, and more than half of all AS-NAGNAG events affected polar amino acid residues. The observed properties of AS-NAGNAG events in plants were similar to those seen in mammals. These results showed that AS-NAGNAG events may provide a mechanism for fine-tuning of DNA-binding proteins in both mammals and land plants. We found 7 gene groups of AS-NAGNAG events that were conserved between Arabidopsis and rice, including 2 groups for RNA-binding proteins. Conservation of the events for RNA-binding proteins is a property also seen in mammals. Furthermore, we found 23 gene groups containing AS-NAGNAG events that occurred in noncorresponding introns of homologous genes. They included 5 groups of DNA-binding proteins, whose number was larger than expected. We think there is a bias with which AS-NAGNAG events are fixed in genes for DNA-binding proteins. Our analysis showed that AS-NAGNAG events found in land plants share similar properties with those in mammals. Based on our results, we propose that AS-NAGNAG events are likely to be a common mechanism in the fine-tuning of protein functions, especially DNA/RNAbinding proteins, in both mammals and plants. Their role might contribute to the construction of complicated transcriptomes and proteomes in the evolutionary history of mammals and land plants