Molecular cloning, expression and exon/intron organization of the bovine β-galactoside α2,6-sialyltransferase gene

In this study, we report the first isolation and characterization of a bovine sialyltransferase gene. Bovine cDNAs prepared from different tissues contain an open-reading frame encoding a 405 amino acid sequence showing 83%, 75%, and 60% identity with human, murine, and chicken ST6Gal I (beta-galactoside side alpha 2,6-sialyltransferase) sequences, respectively. When transfected into COS-7 cells, a recombinant enzyme was obtained which catalyzed the in vitro alpha 2,6-sialylation of Lac-NAc (NeuAc alpha 2-6Gal beta 1-4GlcNAc) and LacdiNAc (NeuAc-alpha 2-6GalNAc beta 1-4GlcNAc) acceptor substrates. The K-m values were 2.8 and 6.9 mM, respectively. Different relative efficiencies (V-max/K-m) for the two precursors (36 for LacNAc and 4.3 for LacdiNAc) were observed. Bovine ST6Gal I gene consists of four 5'-untranslated exons E(-2) to E(1), and five coding exons from E(2) to E(6). This later carries a 3'-untranslated region of 2.7 kb, Gene sequence spans at least 80 kb of genomic DNA. Two processed pseudogenes have been identified. They are 94.3 and 95.6% similar to the bovine cDNA, respectively. Three families of mRNA isoforms were isolated. They differed by their 5'-untranslated regions and could be generated by three tissue-specific promoters. Family 1 is made up of exons E(-2) and E(1) to E(6), family 2 of exons E(-1) to E(6), and family 3 of exons E(1) to E(6). Tissular distribution of transcript families appears noticeably different than those described in human and rat.