Mapping of transcription sites of simian virus 40-specific late 16S and 19S mRNA by electron microscopy.

Simian virus 40 (SV40) late 19S and 16S mRNAs were annealed to complementary regions of partially melted viral double-stranded SV40(LHpa II) DNA or SV40(LBam HI) DNA. The RNA-DNA hybrid regions within the DNA molecules were visualized as loops [with SV40(LBam HI) DNA] in the electron microscope. The data confirm the previous localizations of the 3' and 5' ends of 16S SV40 mRNA and of the 3' end of late 19S SV40 mRNA. The 5' end of the major stable SV40 late 19S mRNA has been positioned at 0.755 map unit. Thus, the sequences of viral DNA from 0.655 to 0.755 map unit, including the replication origin, are not converted into major stable species of late viral mRNA.