Complete genomic organization of futb encoding a bovine alpha 3-fucosyltransferase: Exons in human orthologous genes emerged from ancestral intronic sequences

The futb gene, which encodes the first bovine alpha 3-fucosyltransferase de scribed, consists of five exons (a, b, c, d, and e), the first four being l ocated upstream of the coding exon e. Together with the four introns (i1, i 2, i3, and i4) they span a DNA genomic sequence of about 10 kb. futb is exp ressed as four tissue-specific transcripts differing by their 5'-untranslat ed (5'-UT) regions, but only one transcript includes all exons, while the o ther three begin at internal sites of exon c. A short sequence of the latte r is homologous to distinct 5'-UT exons of FUT6 (alpha 3-fucosylation) and FUT3 (alpha 4-fucosylation), two human genes whose coding sequences are hom ologous to coding exon e of futb. Upstream and downstream, the exon c intro nic regions of the bovine gene are homologous to 5'-UT exons of human FUT3 (exon B) and FUT6 (exons A, B, and C). Thus, exon c appears to be the most ancestral 5'-UT exon known among these alpha 3-fucosyltransferase genes. In terestingly, distribution of short interspersed nuclear elements in the i3 intron adjacent to exon c reveals that two repeat sequences are joined to f orm a reverse-transcriptase-like encoding sequence highly homologous to an open reading frame located at the 3' end of the bovine gamma globin gene. T his organization suggests that duplication events that have generated the p rimate FUT3-FUT5-FUT6 cluster might have occurred through a long-interspers ed-nuclear-element-based mechanism of unequal crossing over, as described f or the globin cluster. Complete organization of the bovine futb gene reveal s that in addition to duplication events, the lineage leading to primate FU T3, FUT5, and FUT6 genes results from rearrangements of intronic sequences which have created for each new gene specific regulatory 5'-UT exonic seque nces.