ALTERNATE PROMOTERS AND ALTERNATE SPLICING OF HUMAN TENASCIN-X, A GENE WITH 5'-ENDS AND 3'-ENDS BURIED IN OTHER GENES

Tenascin-X (TN-X) is an extracellular matrix protein encoded by a larg e gene that overlaps the steroid 21-hydroxylase (P450c21) gene in the HLA locus on chromosome 6p21.3. This may be the most complex locus in the human genome identified to date, containing 13 overlapping transcr iption units in 160 kb of DNA, Previous studies determined the sequenc e of 39 TN-X exons, encoding a 12 kb open reading frame, but the promo ter(s) of the gene had not been located, We identify the principal TN- X promoter and a previously unknown 5' untranslated exon that lies mor e than 10 kb upstream from the previously known exons, This promoter, which is substantially different from the promoter for TN-C, initiates transcription in human fetal adrenal and muscle, but expression in hu man NCI-H295 adrenocortical carcinoma cells is initiated by two other promoters lying further upstream, One of these is the same as the prom oter for a recently identified Creb-related protein (Creb-rp), but tra nscripts initiated from this promoter in human adrenal NCI-H295 tumor cells are spliced differently from Creb-rp, and are largely retained i n the nuclei of these cells, By analogy with the other two members of the tenascin family, TN-C and TN-R, it has been predicted that TN-X sh ould undergo alternate splicing in its fibronectin-like domains, RACE cloning and RNase protection experiments reveal no such alternate spli cing, The TN-X gene appears to be unique in having both its 5' and 3' ends buried in other genes.