Sequence heterogeneity at the 5' termini of late simian virus 40 19S and 16S mRNAs.

Abstract
The 5''-cap-containing leader sequence of the most abundant 19S and 16S mRNA of SV-40 was previously mapped between 0.67 and 0.76 map units. The 2 late mRNA species contain multiple 5'' ends. Eight different RNase T2-resistant cap structures were identified: m7GpppmAmpU (47%); m7CpppmAmpUmpU (19%); m7GpppmAmpC (16%); m7GpppmAmpCmpA (5%); m7GpppmAmpG (6%); m7GpppGmpC (3%); m7GpppmAmGmpA (2%); m7GpppGmpCmpG (2%). Capped Tl oligonucleotides of 19S and 16S mRNA were isolated by 2 different procedures: chromatography on a DEAE-cellulose column followed by paper electrophoresis and 2-dimensional electrophoresis/homochromatography. Cap structures of the isolated 5'' oligonucleotides were identified. Each of the major caps was associated with a few different 5'' oligonucleotides, implying a vast heterogeneity at the termini of SV-40 late mRNA. On SV-40 DNA, RNA polymerase II has a repertoire of initiation points. In most of the cases, initiation takes place with adenosine triphosphate followed by a pyrimidine. Alternatively, transcription may start at 1 specific point but a unique mechanism of processing generates heterogeneous populations of termini with a common 5'' ATP.