Unique genomic structure and expression of the mouse alpha 2,8-sialyltransferase (ST8Sia III) gene

The mouse Sia alpha 2,3Gal beta 1, 4GalNAc alpha 2,8-sialyltransferase (ST8Sia III) genomic gene, whose transcripts are only expressed in fetal and newborn brain and testis, was isolated and its 5'-flanking region was analyzed. The gene was found to span about 8 kb and to be composed of only four exons. The genomic ST8Sia III gene is much smaller and its organization much simpler than other sialyltransferase genes so far reported, which span more than 25 kb and comprise seven or more exons. In particular, the sialyl motif L of ST8Sia III, which is a highly conserved region in all cloned sialyltransferases, was in one exon. In contrast, this motif is encoded by discrete exons in the other sialyltransferases. The ST8Sia III gene was highly expressed in the mouse brain and gave rise to at least three transcripts (2.1 kb, 2.4 kb, and 6.5 kb), which differed in the length of their 3'-untranslated regions through the alternative use of different polyadenylation sites. Primer extension and S1 nuclease protection analyses of mRNA prepared from newborn brain revealed that ST8Sia III gene expression started from a unique site at 382 nt upstream of ATG. Although the promoter region lacked an apparent TATA or CCAAT box and potential regulatory motifs, a transfection experiment involving neuroblastoma cells expressing ST8Sia III demonstrated the minimal promoter activity exhibited by the proximal region 418 bp upstream from the ATG codon, which suggests the presence of tissue-specific enhancer elements.