Identification of the transcription start site of the human GPV gene. (A) Primer extension mapping of the 5′-ends of GPV transcripts from Dami cells and platelets (numbering according to Lanza et al45). (Bold arrowhead) start site common to platelet and Dami RNA; (open arrowheads) additional upstream sites in platelet RNA. (B) Positions of the primers used for PCR mapping are indicated by arrows: PM1 (nt 1433 to 1450), PM2 (nt 1411 to 1428), PM3 (nt 1376 to 1393), and PM4 (nt 2583 to 2566). The intron sequence is in lowercase characters and the GPV intron donor and acceptor splice sites are in bold characters. (C) RT-PCR mapping of the 5′-ends of GPV transcripts. The primer pairs defined in (B) were tested on Dami total RNA and platelet poly(A)⁺ RNA and neg corresponds to RT-PCR without reverse transcriptase. PCR products were identified by 2% agarose gel electrophoresis and ethidium bromide staining.