THE SYNAPTOPHYSIN-ENCODING GENE IN RAT AND MAN IS SPECIFICALLY TRANSCRIBED IN NEUROENDOCRINE CELLS

Synaptophysin (SY) is an integral membrane protein of presynaptic small (30-80-nm) translucent vesicles also present in dispersed neuroendocrine cells. As the occurrence of this type of vesicle is specific for two major pathways of differentiation, the neuronal and neuroendocrine-epithelial information on the regulation of SY synthesis should contribute to an understanding of regulatory principles common to both pathways. Isolation and comparison of the complete rat and human single-copy genes showed that despite the difference in size (16 kb in rat vs. 13 kb in man) intron/exon boundaries are precisely conserved. Surprisingly, intron VI is located in the 3'-noncoding region in both species. The major transcriptional start point, as determined by primer extension and Sl-nuclease protection analyses in rat pheochromocytoma-derived PC12 cells and rat brain, mapped to a site 27 nt 5' of the first methionine codon. Unexpectedly, the 5' upstream region is devoid of any TATA or CAAT boxes, but shows instead typical features of 'housekeeping' genes, i.e., G + C-rich islands and four Sp 1-binding motifs. Using 'nuclear run-on' assays, we have identified examples in which SY synthesis is regulated at the transcriptional level. Reporter gene constructs showed that approx. 1.2 kb of the immediate upstream region contains promoter/enhancer elements that were, however, insufficient to confer cell-type specific expression, whereas sequences farther upstream were able to suppress thymidine kinase promoter activity in a cell-type-dependent fashion.