The 16 S mRNA that directs the synthesis of VP1, the major structural protein of Simian virus 40 (SV40), is composed of a leader sequence of 203 nucleotides joined to a transcript that contains all the codons for VP1. The leader is transcribed from DNA between 0.723 and 0.762 map units while the VP1 coding transcript is copied from DNA from 0.94 to 0.17 map units. The leader sequence joins the transcript of the coding region at a position 42 nucleotides upstream from the initiator codon for VP1 by a 3':5'-phosphodiester linkage. Longer leader sequences also occur in a portion of SV40 late mRNA.