REGULATION OF N-LINKED GLYCOSYLATION - NEURONAL CELL-SPECIFIC EXPRESSION OF A 5' EXTENDED TRANSCRIPT FROM THE GENE ENCODING N-ACETYLGLUCOSAMINYLTRANSFERASE-I

A key transferase in the generation of mature N-linked carbohydrates i s the medial Golgi enzyme N-acetylglucosaminyltransferase I (EC 2.4.1. 101; GlcNAc-TI) which is encoded by the Mgat-1 gene. Previous studies revealed two size classes of Mgat-1 mRNA that are differentially expre ssed in mouse tissues. Nearly all tissues possess a shorter form (simi lar to 2.9 kb), whereas brain has predominantly a longer Mgat-1 mRNA ( similar to 3.3 kb). We now show, by in situ hybridization of horizonta l sections of adult mouse brain, that Mgat-1 RNA levels vary markedly in different brain cell types with the greatest expression being in ne uronal cells. The differential expression of the similar to 2.9 kb and similar to 3.3 kb Mgat-1 mRNAs is likely to be controlled by tissue-s pecific promoters since the size difference between the mRNAs was foun d to reside entirely in the 5' untranslated region of the similar to 3 .3 kb mRNA. Evidence for this was obtained by an RNase H strategy, rev erse transcription-polymerase chain reaction (RT-PCR) and 5' anchored PCR. All of the 0.4 kb difference in size was localized upstream of th e previously isolated cDNA sequence. Sequence information from this re gion was obtained from a mouse brain cDNA library by a PCR amplificati on strategy and a probe specific for the 3.3 kb mRNA was generated. Th is probe hybridized uniquely to the similar to 3.3 kb Mgat-1 mRNA and Southern blot analysis showed that the new sequence is physically link ed to the Mgat-1 gene. A tissue culture model which displays an increa se in expression of the similar to 3.3 kb Mgat-1 mRNA transcript durin g differentiation to neuronal cells has been developed in P19 embryona l carcinoma (EC) cells. The combined data suggest that 5' exons in the Mgat-1 gene are differentially utilized by tissue-specific promoters and that transcription factor(s) which specify production of the simil ar to 3.3 kb Mgat-1 mRNA are induced by retinoic acid treatment of P19 EC cells.