Direct repeats found in the vicinity of intron splice sites

Four main classes of introns (group I, group II, spliceosomal, and archaeal) have been reported for all major types of RNA from nuclei and organelles of a wide range of taxa. When and how introns inserted within the genic regions of genomes, however, is often unclear. Introns were examined from Archaea, Bacteria, and Eukarya. Up to 80 bp surrounding each of the 5' and 3' intron/exon borders were compared to search for direct repeats (DRs). For each of the 213 introns examined, DNA sequence analysis revealed DRs at or near the intron/exon borders, ranging from 4 to 30 bp in length, with a mean of 11.4 bp. More than 80% of the repeats were within 10 bp of the intron/exon borders. The numbers of DRs 6-30 bp in length were greater than expected by chance. When a DNA segment moves into a new genomic location, the insertion involves a double-strand DNA break that must be repaired to maintain genome stability and often results in a pair of DRs that now flank the insert. This insertion process applies to both mobile genetic elements (MGEs), such as transposons, and to introns as reported here. The DNA break at the insertion site may be caused by transposon-like events or recombination. Thus, introns and transposons appear to be members of a group of parasitic MGEs that secondarily may benefit their host cell and have expanded greatly in eukaryotes from their prokaryotic ancestors.