Gene organization of bovine BCNT that contains a portion corresponding to an endonuclease domain derived from an RTE-1 (Bov-B LINE), non-LTR retrotransposable element: Duplication of an intramolecular repeat unit downstream of the truncated RTE-1

BCNT (a protein named after Bucentaur or craniofacial development protein 1) has a unique structure in Ruminantia. Bovine BCNT contains a region of the endonuclease domain derived from a truncated RTE-1 (previously called Bov-B LINE), a non-LTR retrotransposable repetitive element, and two repeat units (intramolecular repeat, IR) each with 40 amino acids in the C-terminal region. In contrast the human and mouse BCNT proteins contain one repeat unit and lack the RTE-1-derived portion. The 3′ UTR of bovine bcnt cDNA also contains an approximately 300-bp portion homologous to the 3′-part of RTE-1. We examined the bovine bcnt genomic DNA sequence to understand how the bovine bcnt gene has been organized. The sequence of 3′ UTR homologous portion was found to more closely resemble the Art2 element than the bovine RTE-1. By PCR screening a bovine/hamster hybrid somatic cell panel, the bovine bcnt gene was mapped to chromosome 18, syntenic human chromosome 16q on which human BCNT is located. The bcnt genomic DNA sequence corresponding to the cDNA downstream of a RTE-1 derived portion reveals that each IR unit is flanked by both 5′-side and 3′-side introns and that 3′-UTR consists of one exon. The alignment of the above sequence with a bovine RTE-1 did not show any significant homology downstream of the endonuclease domain. On the other hand, the alignment of the intron sequences with each other revealed that the six sequential homologous segments ranging in size from 40 to 453 bp existed over a 1 kb long sequence between both the 5′- and 3′-side introns flanking each bovine IR unit. In addition, both the 174-bp of 5′-side intron and 80-bp of 3′-side intron neighboring each 120-bp IR exon are significantly homologous among the two bovine IRs, human IR and mouse IR. These results suggest that a truncated bovine RTE-1 was inserted into the intron upstream of an IR unit of an ancestor bcnt gene and that a duplication of a relatively long region that includes IR occurred in the bovine genome.