A single-nucleotide exon found in Arabidopsis

Most eukaryotic genes carry protein-coding exons that are separated by non-coding introns(1,2). Pre-mRNA splicing is performed by the spliceosome, a large ribonucleoprotein complex comprised of five small nuclear ribonucleoproteins (snRNPs U1, U2, U4, U5 and U6) and a large number of associated proteins(3,4). The size of introns ranges from 13 to over 300,000 nucleotides(5,6). Sufficient evidence suggest that intronic sequences not only determine the splicing pattern(7), but also have regulatory functions in gene expression(8). Although most known regulatory sequences including the conserved GT and AG located at the beginning and the end of introns, respectively, an A at the branch point and a pyrimidine tract in spliceosome-binding and intron splicing are located in introns(9), exonic sequences play an important role in accurate splicing as well(10-12). The average size of exons is approximately 130 nucleotides in vertebrates, and 180 nucleotides in plants(13). Studies have showed that exons with less than 51 nucleotides may cause exon skipping, and exons that are too small in size may hinder the recognition of adjacent spliceosome binding(14-17). However, internal micro-exons with less than 25 nucleotides have been identified in different eukaryotic organisms by sequencing and computational analyses(18,19). The smallest naturally available exon that has been experimentally characterized so far has 3 nucleotides(16,20). Here we report the identification of a single-nucleotide exon in Arabidopsis