We studied synthesis of viral and cellular RNA in the presence and absence of 5-fluorodeoxyuridine (FdU, an inhibitor of DNA synthesis) during lytic infection with polyoma virus in confluent, primary mouse kidney cell cultures. In the presence of FdU, synthesis of early 19S polyoma mRNA and of polyoma tumor (T)-antigen, i.e. expression of the early viral gene, is rapidly followed by a mitogenic reaction of the host cell; it leads to an increase of 30 +/- 5% in cellular, mainly 28S and 18S rRNA, followed by activation of the cellular DNA-synthesizing apparatus. Polyoma-induced cellular RNA synthesis is paralleled by increased production of early 19S mRNA and begin of expression of the late viral genes, leading to synthesis of small amounts of late 19S and 16S mRNAs. Changed expression of the viral genome occurs in the absence of detectable synthesis of polyoma DNA I. Infection in the absence of FdU induces the same sequence of events; it is followed, however, by duplication of the mouse cell chromatin (S-phase) and production of progeny virus.

Heterogeneous polyoma giant RNA molecules have been isolated by oligo(dT)- cellulose chromatography during the late phase of a lytic cycle of infection of mouse kidney cell cultures. These RNAs have sedimentation coefficients in denaturing Me2SO gradients that are greater than 26S and thus apparently correspond to RNA molecules larger than one strand of polyoma DNA. Approximately 15% of total nuclear polyoma late giant RNAs contained tracts of poly(A) and were retained by oligo(dT)-cellulose. The polyoma late giant RNAs as well as heterogeneous nuclear RNAs (HnRNAs) were found to have a slightly lower sedimentation rate in Me2SO-chloral hydrate density gradients than sedimentation values in sucrose gradients indicated. Even when synthesis of viral DNA and the production of capsid protein are blocked by 5-fluorodeoxyuridine (FdU), 10% of polyoma-specific RNA (as determined by sedimentation analyses under aqueous conditions) was shown to contain tracts of poly(A). In contrast to our findings on polyoma late giant RNA, nuclear polyoma RNA synthesized in the presence of FdU sedimented in denaturing Me2SO-chloral hydrate gradients considerably slower (from 15 to 30S) in relation to HnRNA and ribosomal precursor RNA. The sedimentation pattern in denaturing Me2SO gradients suggest that Py RNA synthesized late in lytic infection in the presence of FdU may be no longer than one transcript of Py DNA.

Using three different polyomavirus hr-t mutants and two polyomavirus mlT mutants, we studied induction of S-phase by mutants and wild-type virus in quiescent mouse kidney cells, mouse 3T6 cells, and FR 3T3 cells. At different times after infection, we measured the proportion of T-antigen-positive cells, the incorporation of [3H]thymidine, the proportion of DNA-synthesizing cells, and the increase in total DNA, RNA, and protein content of the cultures. In permissive mouse cells, we also determined the amount of viral DNA and the proportion of viral capsid-producing cells. In polyomavirus hr-t mutant-infected cultures, onset of host DNA replication was delayed by several hours, and a smaller proportion of T-antigen-positive cells entered S-phase than in wild-type-infected cultures. Of the two polyomavirus mlT mutants studied, dl-23 behaved similarly to wild-type virus in many, but not all, parameters tested. The poorly replicating but well-transforming mutant dl-8 was able to induce S-phase, and (in permissive cells) progeny virus production, in only about one-third of the T-antigen-positive cells. From our experiments, we conclude that mutations affecting small and middle T-antigen cause a reduction in the proportion of cells responding to virus infection and a prolongation of the early phase, i.e., the period before cells enter S-phase. In hr-t mutant-infected mouse 3T6 cells, production of viral DNA was <10% of that in wild-type-infected cultures; low hr-t progeny production in 3T6 cells was therefore largely due to poor viral DNA replication.

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The polyoma mRNAs present in the cytoplasm of primary cultures of mouse kidney cells during lytic infection were characterized by sedimentation velocity analysis and by hybridization to polyoma DNA fragments generated by a specific endonuclease of Hemophilus parainfluenzae (Hpa II). Three distinct species of polyoma mRNAs were found and mapped on the viral genome: (a) early 19S polyoma mRNA, which is complementary to about 45% of the early strand and maps from approximately 78 (5' end) to 23 (3' end) MATERIALS AND METHODS Cultures of mouse kidney cells (1, 7) were infected 2 days after confluence with 0.4 ml of a suspension of wild type, plaquepurified polyoma virus containing 5 X 108 plaque-forming units (PFU)/ml.Purified polyoma DNA 1 (8) was digested with Hpa II, and the fragments were separated by polyacrylamide gel electrophoresis (9). They were eluted from the gel electrophoretically into dialysis bags and concentrated by ethanol precipitation.Polyoma DNA I was denatured and fixed on cellulose filters (30 mm) as described (10). Hpa II fragments were denatured in 0.2 M NaOH for 10 min at room temperature, neutralized with acetic acid, and fixed on filters by the same method. The 30-mm filters were cut into 25 square filters of 3.5 mm, each carrying 0.2 ,ug of polyoma DNA or 0.05-0.1 ug of an Hpa II fragment. These quantities assured DNA excess for hybridization. Using radioactive Hpa II fragments, we found that 90% of the input DNA of fragments 1, 2, and 4 were fixed on the filters, but only 65% of fragment 6.Early RNA was labeled from 8 to 11 hr after infection with