Bacteriophage T5 DNA was examined in an electron microscope after limited digestion with exonuclease III from Escherichia coli. The effect of the exonuclease treatment was to convert each naturally occurring single-chain interruption in T5 DNA into a short segment of single-stranded DNA. The locations of these segments were determined for T5st(+) DNA, T5st(0) DNA, and fragments of T5st(0) DNA generated by EcoRI restriction endonuclease. The results indicate that single-chain interruptions occur in a variable, but nonrandom, manner in T5 DNA. T5st(+) DNA has four principal interruptions located at sites approximately 7.9, 18.5, 32.6, and 64.8% from one end of the molecule. Interruptions occur at these sites in 80 to 90% of the population. A large number of additional sites, located primarily at the ends of the DNA, contain interruptions at lower frequencies. The average number of interruptions per genome, as determined by this method, is 8. A similar distribution of breaks occurs in T5st(0) DNA, except that the 32.6% site is missing. At least one of the principal interruptions is reproducibly located within an interval of 0.2% of the entire DNA.

The bacteriophage T5 is known to spontaneously generate deletion mutants (st mutants) exhibiting enhanced resistance to heat inactivation in citrate buffer. A series of such mutants has been isolated and the deletions visualized by electron microscopy of heteroduplex molecules. The deletions are found to cluster in one region of the chromosome.

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We purified the p19 proteins from the Prague C strain of Rous sarcoma virus, avian myeloblastosis virus, B77 sarcoma virus, myeloblastosis-associated virus-2(0), and PR-E 95-C virus and measured their binding affinities for 60S viral RNA by the nitrocellulose filter binding technique. The apparent association constants of the p19 proteins from Rous sarcoma virus Prague C, avian myeloblastosis virus, and B77 sarcoma virus for homologous and heterologous 60S RNAs were similar (1.5 x 1011 to 2.6 x 1011 liters/mol), whereas those of myeloblastosisassociated virus-2(0) and PR-E 95-C virus were 10-fold lower. The sizes and relative amounts of the virus-specific polyadenylic acid-containing RNAs in the cytoplasms of cells infected with Rous sarcoma virus Prague C, myeloblastosisassociated virus-2(0), and PR-E 95-C virus were determined by fractionating the RNAs on agarose gels containing methylmercury hydroxide, transferring them to diazobenzyloxymethyl paper and hybridizing them to a 70-nucleotide complementary DNA probe. In cells infected with Rous sarcoma virus Prague C we detected 3.4 x 106-, 1.9 X 106_, and 1.1 x 106-dalton RNAs, in PR-E 95-C virusinfected cells we detected 3.4 x 106_, 1.9 x 106_ and 0.7 x 106-dalton RNAs, and in cells infected with myeloblastosis-associated virus-2(0) we detected 3 x 106_ and 1.3 x 106-dalton RNAs. Each of these RNA species contained RNA sequences derived from the 5' terminus of genome-length RNA, as evidenced by hybridization with the 5' 70-nucleotide complementary DNA. The ratios of subgenomic mRNA's to genome-length RNAs in cells infected with myeloblastosis-associated virus-2(0) and PR-E 95-C virus were threeto five-fold higher than the ratio in cells infected with Rous sarcoma virus Prague C. These results suggest that more processing of viral RNA in infected cells is correlated with lower binding affinities of the p19 protein for viral RNA, and they are consistent with the hypothesis that the p19 protein controls processing of viral RNA in cells. Several investigators have presented evidence that there are at least three viral mRNA's in the cytoplasm of avian oncornavirus-infected cells, which have sedimentation coefficients of 38S, 28S, and 22S (3, 5, 10, 32). Hybridizations of complementary DNA (cDNA) probes representing specific regions of the viral genome have shown that the 38S RNA represents the entire viral genome, the 28S RNA represents the envelope and sarcoma genes, and the 22S RNA represents the sarcoma gene. The first RNA serves as the mRNA for the group-specific antigens and the ,B chain of reverse transcriptase (7, 8, 14, 16, 17), the second RNA serves as the mRNA for the envelope glycoprotein (5, 18, 28, 30), and the third RNA may serve as the mRNA for the sarcoma protein (2, 19, 20). The 28S and