Rapidly labeled RNA was extracted from monkey cells after infection with Simian Virus 40 (SV40) and exposure to short pulses of [5-3Hjuridine late in infection. When this RNA was self-annealed, it became resistant to digestion with ribonuclease. The fraction of RNA that resisted the ribonuclease treatment decreased with increased labeling time, or when a short pulse of radioactivity was followed by incubation with unlabeled uridine and actinomycin D. The RNase-resistant RNA was isolated by chromatography on Sephadex G-100 and shown to be double-stranded by its susceptibility to ribonuclease as a function of salt concentration and temperature. This behavior was not due to RNA-DNA hybrid formation, since deoxyribonuclease had no effect upon the double-stranded molecules, even after their denaturation. The relation of the double-stranded RNA to SV40 was demonstrated by the hybridization of about 50%7 (corrected value, >90%7,) of the separated RNA strands with component I of SV40 DNA from plaque-purified virus. After self-annealing in formamide at low temperature, about 10%O of the rapidly labeled, viral RNA sedimented at 13 S. This value corresponds in size to about 60%/c of the SV40 DNA.These observations indicate that late in infection of monkey cells, SV40 DNA is transcribed symmetrically over a considerable portion of its length, and that subsequently some sequences from one or both of the RNA strands are degraded.It has been generally accepted that transcription is asymmetric, in the sense that only one of the two DNA strands of a given gene serves as a template for RNA (1). However, the possibility that both DNA strands are transcribed, and that some sequences from one or both of the RNA strands are subsequently degraded, has not been excluded. Indeed, it has recently been found that transcription of mitochondrial DNA in HeLa cells operates according to the latter possibility (2. 3). SV40 DNA has circular configurations similar to those of mitochondrial DNA from HeLa cells (4). Early after infection, and in SV40-transformed cells, the viral genome is only partially transcribed, while at later stages in the infection process most or all of the genetic information contained in the SV40 genome is transcribed (5, 6). I show that late after infection of monkey cells with SV40, the viral DNA is transcribed symmetrically over a considerable portion of its length and, subsequently, some sequences from one or both of the RNA strands are degraded.
MATERIALS AND METHODSGrowth of SV40 (strain 777) not purified by plaque formation on monkey cells of the BS-C-1 line, as well as concentration and purification of the virus from the tissue culture lysates, were described (5). In all experiments, cultures (4 X 106 cells) were infected with 1.0 ml of the same stock of SV40 [4 X 108 plaque-forming units (PFU)/ml] except in the experiment described in Table 2, where both plaque-purified and nonpurified virus was used for infection of the cells. DNA was extracted from the purified virus with phenol (7) In each case, RNA...