A transposable element-mediated gene divergence that directly produces a novel type bovine Bcnt protein including the endonuclease domain of RTE-1

Ruminant Bent protein with a molecular mass of 97 kDa (designated p97Bcnt) includes a region derived from the endonuclease domain of a retrotransposable element RTE-1. Human and mouse Bcnt proteins lack the corresponding region but have a highly conserved 82-amino acid region at the C-terminus that is not present in p97Bcnt. By screening a bovine BAC library, we found two more bcnt-related genes: human-type bent (h-type bent) and its processed pseudogene. Whereas the pseudogene is localized on chromosome 26, both bcnt(p97) and the h-type bent genes are found on bovine chromosome 18, a synteny region of human chromosome 16 on which human BCNT is localized. Complete nucleotide sequencing of the BAC clone reveals that the bcnt(p97) and h-type bent genes are located just 6 kb apart in a tandem manner. The two h-type bent and bcnt(p97) genes are active at both the transcriptional level and the protein level. H-type bovine Bent is more like human BCNT than p97Bcnt, when compared at their N-terminal regions. However, phylogenetic analysis using the N-terminal region of the bent gene family revealed that the duplication of bovine genes occurred within the bovine lineage with significantly accelerated substitution in bcnt(p97). This acceleration was not ascribed definitely to positive selection. After duplication, one of the bovine bent genes recruited the endonuclease domain of an intronic RTE-1 repeat accompanied by the accelerated substitution at the 5'-ORF, resulting in creation of a novel type of Bcnt protein in bovine.