Evidence for post-transcriptional selection of viral mRNA in cells transformed by human adenovirus 12

Transport of functional messenger RNA from liver nuclei in a reconstituted cell-free system

The reliability of a reconstituted cell-free system for messenger RNA processing and transport, consisting of isolated nuclei in fortified cytosol, has been evaluated in terms of the functionality and regulated release of the transported product. The poly(A) messenger RNA transport in vitro formed appropriate initiation complexes with ribosomes in an optimized translation system and had template activity comparable to that transported in vivo. The intra-nuclear origin of this messenger RNA is supported by pulse-labeling studies, its transport from detergent-treated nuclei and the absence of the release under non-transport conditions. Serum albumin was identified by immunoprecipitation and electrophoresis as one of the products synthesized when the transported RNA was translated in vitro. The transport of messenger RNA in the cell-free system was dependent on specific cytosol (soluble cytoplasmic) proteins. These proteins, which constitutes less than 0.1% of the total cytosol proteins, are precipitated wtih streptomycin with high specificity.

Nick translation of mammalian DNA

The labelling of mouse DNA by nick translation with DNA polymerase I has been investigated with respect to the time of incubation, requirement for DNAase I, size of the product, and uniformity of labelling, and the hybridisability and stability of the resultant labelled probes. Total mouse DNA and reannealed unique mouse DNA sequences can be labelled by nick translation in the presence of [3H]dCTP and [3H]TTP to a specific activity of 7 . 10(6)--20 . 10(6) cpm/microgram DNA. The hybridisation characteristics of nick-translated whole DNA with an excess of unlabelled mouse-embryo driver DNA indicates that no preferential labelling of repetitive or unique DNA sequence classes occurs. In addition, the proportion of unique DNA sequences labelled by nick translation which hybridises with polyadenylated nuclear RNA from Friend cells is the same as that of unique DNA sequences isolated from cells labelled with [3H]thymidine in vivo, indicating that few (if any) of the unique DNA sequences are unrepresented in the nick-translated probe. Probes which contain [3H]dTMP are unstable, and show a considerable reduction in hybridisability over a period of 6 months at --20 degrees C. The decrease is accompanied by an increase in the number of mismatched sites in duplexes containing the labelled probe (as shown by thermal stability measurements of hybrid molecules) and a decrease in the rate of hybridisation of the probe with total mouse DNA. In contrast, DNA which is labelled with [3H]dCMP alone is stable, and does not show any decrease in hybridisability on prolonged storage.

Adenovirus type 2 early nuclear and mRNA: kinetic estimation of l anf r DNA strand fractions complementary to different abundance classes of viral RNA

RNA from unfractionated cells, nuclei, and polyribosomes was extracted from adenovirus 2 (Ad2)-infected KB cells early in infection and annealed in vast excess in liquid to purified Ad2 l (heavy) and r (light) [(32)P]DNA strands (specific activity, 3 x 10(6) to 1.5 x 10(7) cpm/mug). The number of abundance classes of Ad2 RNA, their relative concentrations, and the strand fraction from which they arose were determined by a computer-assisted nonlinear regression analysis of hybridization kinetic data. Whole-cell RNA and nuclear RNA annealed to 60 and 40%, respectively, of l and r strands. Well-defined abundance (kinetic) classes were identified: abundant and scarce classes were complementary to 15 to 17 and 40 to 45%, respectively, of l strand, and to 11 to 16 and 17 to 23%, respectively, of r strand. In whole-cell RNA and nuclear RNA the abundant classes were 57 to 208 and 13 to 27 times more concentrated, respectively, than scarce classes. RNA-RNA hybrids were isolated that annealed to about 70% of both strands, indicating that whole-cell RNA and nuclear RNA hybridization values were minimal. Polyribosomal RNA appeared to anneal as three abundance classes to each DNA strand; abundant, scarce, and very scarce classes, respectively, hybridized to 6, 5, and about 10% of l strand and 7 (6 to 8), 10 (8 to 13), and about 19% of r strand. The abundant classes were 41 (11 to 67) times more concentrated than the scarce classes and 10(3) times more concentrated than the very scarce classes. Although the biological significance of these classes is not known, the very scarce classes probably represent nuclear RNA contaminants of polyribosomal RNA. The abundant and scarce classes may comprise mRNA, because together they are complementary to about the same fraction of each DNA strand (11% [10 to 12%] and 17% [14 to 20%] of l and r strands) known to be expressed as early mRNA. Thus, nuclear RNA contains Ad2 RNA sequences not found on polyribosomes; most or all of both DNA strands are transcribed, but only certain transcripts are processed into mRNA. It is not known whether "non-mRNA" transcripts are intermediates in the pathway of early mRNA production.

Spontaneous mutations in ageing human cells: studies using a Herpesvirus probe

Several fundamental theories of ageing postulate that spontaneous mutations increase in somatic cells as they age. This has not until recently been directly tested in mammalian cells, because of the problem of estimating mutation rate in declining cell populations. One possible approach is to infect cells with lytic virus and measure the mutation frequency in virus made by the cellular machinery. Three temperature-sensitive mutants of Herpesvirus were used to infect human fibroblasts in vitro. The reversion frequency was examined in virus harvested from young and old cells. It was found that with tsE there was a hundredfold increase in reversion rate in old compared to young cells, with tsD the rates were roughly similar and with tsG a fortyfold decrease in old cells. These differences are not due to differential rates of virus production. It is proposed that errors in specific functions in old cells influence the spontaneous mutation rate of the three viral mutants in opposite directions. It is pointed out that certain unknown factors limit the usefulness of viral probes in ageing research and cast doubt on previous work of this type.