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A fraction of the unintegrated viral DNA that appears early after infection of mouse cells by Moloney leukemia virus is infectious. The infectivity could be deomonstrated by an XC plaque assay of the cells exposed to DNA co-precipitated with calcium phosphate. The number of plaques deriving from closed-circular, supercoiled DNA was proportional to the concentration of added DNA, indicating that a single DNA molecule of about 5.5 X 10(6) daltons carries all the viral information. Nonsupercoiled viral DNA is also infectious; these molecules appear to be largely double-stranded and 5 to 6 X 10(6) dalthons in mass.

32P-labelled derived 40-S ribosomal subunits and 80-S monoribosomes entered the polyribosome fraction of a rabbit-reticulocyte lysate active in protein synthesis by a process that was dependent on time, energy and temperature. Both 40-S and 60-S subparticles from the labelled monoribosome entered the polyribosomes at the same rate. The addition of excess unlabelled 40-S subparticles to the cell-free system containing labelled monoribosomes increased the ratio of 60-S to 40-S-subparticle radioactivity in the polyribosome fraction. This indicated a competition between added unlabelled and monoribosome-derived 40-S subparticles for entry into polyribosomes. From the evaluation of the translation time of mRNA, of the rate of monoribosome-polyribosome exchange, and of the equilibrium distribution of the cycling labelled monoribosomes and unlabelled subparticles with polyribosomes it is concluded that the dissociation of single ribosomes into subunits is an obligatory step for their entry into the polyribosoma1 fraction and that this process is coupled with initiation of mRNA translation.

SEVERAL hours after infection of cells by RNA tumour viruses new, free viral DNA molecules can be detected1–5 which are apparently intermediate between the initial RNA-dependent DNA transcription product and the final integrated viral genome1,6. Work on Rous sarcoma virus6,7 and Moloney murine leukaemia virus (Mo-MuLV)4 showed that this DNA is resolvable by ethidium bromide–caesium chloride (EB–CsCl) centrifugation into two classes of molecule. The first, from the lower band of the EB–CsCl gradient, contains closed-circular, double-stranded, super-coiled viral DNA molecules of molecular weight 5×106–6×106, about twice the weight of the single-stranded subunits isolated from virion RNA. The kinetics of disappearance of these DNA molecules suggest that they are the precursors of viral genomes integrated into cellular DNA5. The second class of viral DNA, from the upper band of the gradient, is characterised here. These molecules have the properties of intermediates between the initially synthesised RNA–DNA hybrids and the double-stranded, supercoiled form of the genome. Careful preparation reveals that these molecules are partially single stranded. The first strand of the viral DNA is complexed with shorter segments of the subsequently synthesised second strand.

THE biological reality of extra-polyribosomal mRNA and its role as a precursor to polyribosomal RNA have been outlined1,2. Although the occurrence of mRNA activity in cytoplasmic postribosomal supernatant from rabbit reticulocytes3,4 has been reported, the active fraction capable of directing globin synthesis has not been identified. We describe here a procedure for the isolation of a free cytoplasmic 10S RNA from rabbit reticulocytes and present evidence that the product synthesized under its direction in a rat liver, cell-free system is globin α-chain. In the same conditions, polyribosome-derived 10S mRNA5 directed the synthesis of both globin α and β-chains.

Infection of JLS V-9 (mouse bone marrow) cells with Moloney murine leukemia virus leads to appearance of virus-specific DNA in the low molecular weight DNA fraction from infected cells. Most of this DNA is double-stranded. Two discrete forms of double-stranded DNA, both of molecular weight 6 times 10-6, have been characterized. One of these forms is closed-circular, supercoiled DNA; the other form is a mixture of nicked-circular and linear DNA.

Haemin was shown to stimulate α but not β globin synthesis by study of the translation of haemoglobin mRNA in Xenopus oocytes. Haemin acted at the translational level in this system.