Efficiency of processing of viral RNA during the early and late phases of productive infection by polyoma virus

The efficiency of processing of polyoma viral RNA and of its export from nucleus to cytoplasm was measured in primary mouse kidney cells by comparing the initial rates of incorporation of [3H]uridine into cytoplasmic and nuclear viral RNA. Appropriate methods of cell fractionation were chosen to maximize yields of cytoplasmic RNA and to minimize leakage of nuclear RNA. Incorporation of [3H]uridine into cellular 4S RNA in the cytoplasm was followed to monitor pool equilibration and maintenance of an excess of radioactive precursor throughout the experimental period. During the early phase of infection (9 to 11 h, in the presence of 5-fluorodeoxyuridine), viral RNA was rapidly and efficiently exported from nucleus to cytoplasm. Viral RNA appeared in the cytoplasm within 6 min of its synthesis, greater than half of the viral RNA synthesized in the nucleus was exported to the cytoplasm. In contrast, during the late phase of infection (28 to 30 h), viral RNA was exported more slowly, appearing in the cytoplasm 12 to 20 min after its synthesis, and much less efficiently-only 5% of late nuclear transcripts was exported. The poor efficiency of processing of late viral RNA may be, in part, a result of (i) the presence in nuclear transcripts of non-mRNA sequences which are removed during processing; (ii) the presence in nuclear transcripts of multiple copies of mRNA sequences, only one of which is incorporated into mature mRNA; and (iii) inefficient polyadenylation of viral nuclear RNA.