Establishment of infection by spleen necrosis virus: inhibition in stationary cells and the role of secondary infection

The relationship of two early events in the establishment of infection by avian retroviruses, the inhibition of viral DNA synthesis in stationary avian cells and the secondary infection which occurs after infection of replicating cells, was investigated. When neutralizing antibody to spleen necrosis virus was used to prevent secondary infection, the amount of unintegrated linear spleen necrosis virus DNA detected was much lower in infected stationary cells than in infected replicating cells. The amount of unintegrated linear spleen necrosis virus DNA in stationary cells was less than one copy per cell even at high multiplicities of infection. Viral DNA synthesis resumed after stimulation of the cells to replicate. The time of this viral DNA synthesis was closely correlated with renewed cellular DNA synthesis. In addition, blocking secondary infection of replicating cells prevented the rate of virus production from reaching the high levels usually associated with a normal productive infection by SNV. Virus production increased if secondary infection was allowed. However, this rise in virus production was not proportional to the amounts of viral DNA integrated after secondary infection.

Characterization of reticuloendotheliosis virus strain T DNA and isolation of a novel variant of reticuloendotheliosis virus strain T by molecular cloning

Reticuloendotheliosis virus strain T (REV-T) is a highly oncogenic avian retrovirus which causes a rapid neoplastic disease of the lymphoreticular system. Upon infection, this virus gives rise to two species of unintegrated linear viral DNA, which are 8.3 and 5.5 kilobase pairs long and represent the helper virus (REV-A) and the oncogenic component (REV-T), respectively. Restriction endonuclease cleavage maps of these two DNA components indicate that REV-T DNA has a large portion of the genome deleted with respect to REV-A DNA and a substitution about 0.8 to 1.5 kilobase pairs long that is unrelated to REV-A DNA. These additional sequences comprise the putative transforming region of REV-T (rel). A chicken spleen cell line transformed by REV-T produced virus which upon infection gives rise to three species of unintegrated linear viral DNA (8.3, 5.5, and 3,3 kilobase pairs). We isolated the proviruses of the 8.3- and 3.3-kilobase pair species from this cell line by cloning in the phage vector Charon 4A. Restriction enzyme mapping showed that the two proviral clones are proviruses of REV-A and a variant of REV-T, respectively. A subclone of the variant REV-T provirus specific for the rel sequences of REV-T was used as a hybridization probe to demonstrate that the rel sequences are different from the putative transforming sequences of Schmidt-Ruppin Rous sarcoma virus strain A, avain myelocytomatosis virus, avian myeloblastosis virus, avian erythroblastosis virus, Abelson murine leukemia virus, and Friend erythroleukemia virus. In addition, the rel-specific hybridization probe was used to identify a specific set of sequences which are present in uninfected avian DNAs digested with several restriction enzymes. The corresponding cell sequences are not arranged like rel in REV-T.

Ribonucleotides in unintegrated linear spleen necrosis virus DNA

The structure of unintegrated spleen necrosis virus DNA was characterized by using various chemical and enzymatic treatments in conjunction with denaturing gels and nucleic acid hybridization probes. Throughout the course of the viral infection, the predominant species of viral DNA was that of a linear double-stranded molecule containing ribonucleotides covalently joined to the DNA. The majority of both - and + strands were continuous. The ribonucleotide linkages appeared to be relatively short, and the base composition and distribution of the ribonucleotide linkages were heterogeneous. On the average, the - strand had fewer of the ribonucleotide linkages than did the strand. Viral DNA containing ribonucleotide linkages was infectious in DNA transfection assays. The structure of spleen necrosis virus DNA was different from that of Schmidt-Ruppin Rous sarcoma virus-D, and mixed infections demonstrated that the observed differences are a result of cis-acting functions.

Deoxycytidine kinase from calf thymus. Substrate and inhibitor specificity

Kinetic constants were determined for 34 nucleoside substrates of deoxycytidine kinase (EC 2.7.1.74) from calf thymus. Substrate efficiency was assessed by the ratio of Vmax to Km. Inhibition constants were determined for 61 nonsubstrate nucleosides or nucleoside analogues. The enzyme was relatively specific for the pentose moiety of nucleoside substrates. beta-D-2'-Deoxyribonucleosides were more efficient substrates than the corresponding beta-D-arabinonucleosides. Unexpectedly, the L isomer of the beta-arabinonucleoside of cytosine was a more efficient substrate than was the D isomer. beta-Cytidine and beta-5-azacytidine were the only beta-D-ribonucleosides studied that had detectable substrate activity. alpha-Cytidine was an inhibitor but not a substrate. Nucleosides containing a variety of sugar moieties other than those mentioned above did not have detectable substrate activity. The enzyme was relatively nonspecific for the base moiety of nucleoside substrates. 2'-Deoxyribonucleosides of a variety of pyrimidines, purines, and other heterocycles were substrates. Cytosine was the most preferred pyrimidine moiety. 5-Substitution, except with fluorine, decreased substrate efficiency with nucleosides of cytosine or uracil. 2-Fluoradenine was the most preferred purine moiety. The effects of various purine ring substituents were interdependent. Nucleosides containing bulky, hydrophobic substituents on either the base or the pentose moiety had no substrate activity but were relatively potent competitive inhibitors. This suggested the presence of a hydrophobic region on the surface of the enzyme near the active site.