- Volume 8, Issue 1, 1970

Mithramycin inhibited RNA synthesis in chick embryo cells in culture almost as efficiently as actinomycin D, although inhibition was considerably delayed. There was no direct effect on cellular protein synthesis. Mithramycin interfered with the multiplication of fowl plague virus (influenza A) but had little effect on the multiplication of Newcastle disease virus (parainfluenza). Ten µg./ml. of mithramycin, when added immediately after infection, preferentially inhibited the synthesis of fowl plague minus strand RNA in culture, but it had only a slight effect on the production of plus strand RNA. The synthesis of virus RNA-dependent RNA polymerase and RNP-antigen was only slightly inhibited, while the production of haemagglutinin and neuraminidase was strongly affected.

Red clover vein mosaic virus (RCVMV) was compared with other viruses, in particular white clover mosaic virus (WCMV) and clover yellow vein virus (CYVV). All three have filamentous particles, those of RCVMV are 645 nm. long and have a sedimentation coefficient of 160s, those of WCMV are 460 nm. long (119s), and those of CYVV between 700 nm. and 800 nm. long (about 140s). RCVMV and WCMV contain about 6% ribonucleic acid with nucleotide compositions of G31.5, A24.1, C22.7, U21.7 and G15.5, A31.8, C26.9 and U25.7% respectively. RCVMV is distantly serologically related to five viruses of the potato virus S group, CYVV to eight viruses of the potato virus Y group, and WCMV to potato virus X.

RCVMV is photoreactivable; after exposure to ultraviolet radiation, preparations of the virus caused more lesions in Chenopodium amaranticolor when the inoculated plants were kept in the light than in darkness; this is the first virus with seemingly rigid helically constructed particles to show photoreactivation.

Chick embryo lethal orphan virus induced both T and virus capsid antigens during cytolytic infection of chick kidney cells. The T antigen was produced relatively early, 8 or 9 hr after infection, while virus capsid antigen was first detected after 15 to 16 hr. The T antigen appeared as a fine granular fluorescence and also as small blobs confined to the cell nucleus. T antigen was detected in virus-infected hamster, mouse and human cells, but was present in very few cells compared with chick kidney monolayers, where almost all cells contained antigen. The virus capsid antigen had a more complex morphology, with fluorescing spots and blobs in the nucleus; it was also detected as a diffuse fluorescence in the cytoplasm later in infection. The production of virus capsid antigen in chick kidney cells was partially inhibited by cytosine arabinoside (100 µg./ml.) or 5-fluoro-2′-deoxyuridine (30 µg./ml.). These inhibitors of DNA viruses had no effect on the production of virus-induced T antigen in chick kidney cells.

No antigenic relationship was detected by immunofluorescence or complement-fixation tests between chick embryo lethal orphan virus and T antigens of adenovirus type 12. In addition, the adenovirus group hexon antigen was not detected in preparations of chick embryo lethal orphan virus by complement-fixation or immunofluorescence tests.

Sendai virus induced fusion of Ehrlich ascites tumour cells but this capacity was decreased by sonic treatment and the product then interfered with virus-induced fusion. Sucrose density gradient fractionation of the sonic product showed that the capacity for fusion resided in intact particles and the interfering effect in fragments of the virus envelope. Such fractionation also showed that haemolytic activity was restricted to intact particles or to large envelope fragments, while haemagglutination is found with large or small fragments. However, envelope fragments, with haemolytic activity, induced fusion in cell monolayers with little or no inhibitory effect. Envelope fragments complexed with antibody lose their capacity to inhibit fusion and show no capacity for fusion of Ehrlich ascites tumour cells.

These findings, and studies by electron microscopy on interactions of envelope fragments to cells, support the hypothesis that the fusion of suspended cells depends on the strength of contact between cells induced by virus components with haemolytic activity.

Twenty-one temperature-sensitive mutants of Semliki Forest virus were classified by measurement of their capacity to make virus RNA. The RNA+ mutants were defective in a very early step in virus multiplication; possibly in the formation of functional polymerase enzyme. The RNA+ mutants investigated were all defective in their capacity to make the virus envelope protein. All the mutants were capable of inducing interferon formation but two different types of induction could be distinguished. At high multiplicities, both classes of mutant and the wild type induced interferon production without requiring virus RNA synthesis, while at lower multiplicities interferon production depended on the synthesis of virus RNA. It was concluded that at high multiplicities the inducer was newly synthesized RNA. However, a number of the RNA+ mutants were less efficient interferon inducers than the wild type, and at low multiplicities synthesised virus RNA but not interferon.