Characterisation of polyoma late mRNA leader sequences by molecular cloning and DNA sequence analysis

The leader sequences of two of the three polyoma virus late mRNAs were characterised by molecular cloning and DNA sequence analysis. A short single-stranded DNA fragment complementary to the 5' end of the body of mVP1 was used to prime cDNA synthesis, and double-stranded cDNA was inserted into a derivative of pAT 153. Analysis of fourteen mVP1 cDNAs and two mVP3 cDNAs allowed the precise determination of the leader-body joints, and demonstrated that the majority of polyoma late leader sequences consist of exact tandem repeats of a 57 nucleotide sequence present only once in the genomic DNA at 66-67 m.u. The sequences in the genomic DNA borderline this unit are typical of those found at RNA splice points. Leader sequences contained on average three to four repeat units. Nuclease S1 mapping of total late mRNA demonstrated that most mRNA 5' ends map heterogeneously in the 50 nucleotides 5' to the repeated sequence unit. The structures of the leader sequences strongly suggest that they are generated by appropriate splicing events from a tandemly repeated transcript of the entire circular viral genome.