5' untranslated sequences determine degradative pathway for alternate PDGF B/c-sis mRNA's.

The 5' untranslated sequence (5' UTS) of the platelet-derived growth factor B (PDGF B/c-sis) mRNA is GC rich, includes AUGs upstream of the translation initiation site, and inhibits translation of downstream heterologous and autologous coding sequences. Its primary sequence has been remarkably conserved through evolution. In this study, we identified two additional PDGF B/c-sis mRNAs expressed in endothelial cells. These were shown by Northern, primer extension, and nuclease protection analyses to differ in extent of 5' untranslated first exon sequence. The predominant of these 5' truncated transcripts was 2.8kb, extended 15nt upstream of the translation start site and was markedly stabilized by cycloheximide and anisomycin, but not puromycin or pactamycin. Cycloheximide increased the half-life of this mRNA approximately 7 fold, without affecting stability of the full length mRNA. Cycloheximide withdrawal caused its abrupt destabilization. The 2.8kb PDGF B/c-sis mRNA lacks translation inhibitory 5' UTS, encodes the same PDGF B protein, appears initiated from within first exon genomic sequences, and is degraded through a process that is sensitive to translation arrest and distinct from that degrading the 3.8kb PDGF B/c-sis mRNA.