The human thrombospondin 3 gene: analysis of transcription initiation and an alternatively spliced transcript.

Thrombospondin 3 (TSP3) is a member of a family of modular, extracellular proteins that have been implicated in a diverse number of important biological processes. To contribute to an understanding of the precise roles of human TSP3, aspects of TSP3 gene transcription have been investigated. The TSP3 gene (THBS3) shares a promoter/intergenic region of 1.4 kb with the divergently transcribed metaxin gene, and the existence of TSP3 transcription initiation sites in the TSP3/metaxin intergenic region was investigated by a PCR procedure. Transcripts were detected which initiate in the intergenic region, up to several hundred bases upstream from the major transcription start site. An alternatively spliced transcript of TSP3 was also detected by the PCR procedure. This includes a new exon, exon A', which replaces exon A. Exon A' is located in the TSP3/metaxin intergenic region, 1 kb 5' of exon A. In addition, transcripts of metaxin were found with extended 5' ends; these overlap the 5' end of the TSP3 alternative transcript. The complexities of TSP3 transcription initiation revealed by this study could contribute to the tissue-specific expression and diverse functions of TSP3.