Characterization of the 5'-terminal structure of simian virus 40 early mRNA's.

RPC-5 reverse-phase chromatography has been used to isolate fragments of simian virus 40 DNA generated by appropriate digestions with restriction endonucleases. Ten specific DNA fragments, mapping successively in a counterclock-wise direction from 0.67 to 0.515 on the simian virus 40 genome, were each hybridized to cytoplasmic mRNA obtained during the early phase of simian virus 40 infection. Primer extension methods with reverse transcriptase were used to characterize the 5' ends of two species of viral mRNA which were fractionated on sucrose gradients. Analysis of the complementary DNA products demonstrated the presence of two different spliced structures of simian virus 40 early mRNA's, both of which had the same 5'-end sequences (AUU), located at residues 18 to 20 on the viral genome. The mRNA for small-t contained a segment 588 bases in length (residues 18 to 605) spliced to residues 672. A 66-nucleotide segment rich in adenine-thymine was spliced out of this mRNA. The mRNA for large-T contained a segment 308 bases in length (residues 18 to 325) which is also spliced to residue 672. A 346-base segment was spliced from this mRNA. The results suggest that there are two levels for control of genetic expression. One would be the regulation of initiation of transcription at a common promoter; the other involves post-transcriptional splicing.