TRANSCRIPTION OF THE BETA-GALACTOSIDE ALPHA-2,6-SIALYLTRANSFERASE GENE IN B-LYMPHOCYTES IS DIRECTED BY A SEPARATE AND DISTINCT PROMOTER

A single human gene, SIAT1, encodes the beta-galactoside alpha 2,6-sia lyltransferase from which multiple mRNA isoforms are generated, In rat , expression of the hepatic mRNA isoform (Form I) has been defined wit h respect to the transcriptional initiation site and promoter region, We show here that a similar hepatic SIAT1 mRNA isoform exists in human , Another human mRNA isoform, a mature B-cell-specific mRNA isoform (F orm 2), was previously reported, Here, we used 5'-RACE and S1 nuclease protection analysis to define the 5'-untranslated region of Form 2 hu man SIAT1 mRNA. We demonstrate conclusively that Form 2 mRNA is initia ted from a point completely distinct from that of Form I mRNA, A numbe r of cis-acting regulatory elements residing immediately 5' of the For m 2 initiation site includes AP-1, AP-2, NF-kappa B, NF-IL6, C/EBP, an d CREB, A TATAA box is also present 29 bp 5' of the transcriptional in itiation site. CAT reporter gene expression from serially-truncated se gments of the 5'-flanking region of the Form 2 initiation site indicat es that the segment between (-)784 and (+)125 was sufficient to promot e high level CAT expression in Louckes, a mature B-cell line, The 5'-f lanking region to the human Form I initiation site is competent in exp ression of CAT upon transfection of the fusion construct into HepG2 a human hepatoma cell line, Cellular specificity of expression is appare ntly retained, Louckes cells expressed CAT efficiently from Form 2 pro moter but only marginally from the Form I promoter, In contrast, CAT e xpression from Form I promoter is more efficient than from the Form 2 promoter in HepG2 cells.