Abstract
The bulk of late virus-specific RNA synthesized in polyoma virus-infected mouse cells is larger than a single strand of poloma DNA. The arrangement of viral nucleotide sequences in these giant polyoma RNAs was studied by electron microscopy of hybrids between purified high molecular weight viral RNA and the HindII-1 fragment of polyoma DNA, which contains 91% of the viral genome. Hybrid molecules containing a short single-stranded gap (corresponding to the 9% of viral sequences not present in HindII-1), flanked by double-stranded regions, were photographed and measured. The majority of hybrid molecules contained no single-stranded loops or branches, showing that all viral sequences are transcribed contiguously and that no nonviral sequences are present in the RNA. Hybrid molecules, containing RNA up to 3.5 times the genome length, had a repeating structure of single-stranded gaps 8% of genome length interspersed with double-stranded regions 89% of genome length, showing that giant polyoma RNAs contain tandem repeats of the nucleotide sequence of the entire viral DNA. A small proportion of hybrid molecules contained single-stranded branches or deletion loops in characteristic positions, indicating that RNA "splicing" may occur on high molecular weight nuclear polyoma RNA.