STUDIES ON THE REPLICATING POOL OF VIRAL DNA IN CELLS INFECTED WITH PSEUDORABIES VIRUS

Quantitative studies on the maturation process of herpes simplex virus type 1 in Vero cells

We examined the time course of viral DNA synthesis, the formation and envelopment of viral nucleocapsids, and the formation of infectious progeny virus in Vero cells infected with herpes simplex virus type 1 (HSV-1). The results showed that the formation of nucleocapsids coincided with the appearance of enveloped particles as well as of infectious progeny virus, although the synthesis of viral DNA took place approximately 2 h prior to the beginning of nucleocapsid formation. These results indicate that the rate-limiting step in the virogenesis of HSV-1-infected cells is the encapsidation of viral DNA and that the enveloped virus is formed immediately after the formation of nucleocapsids and is infectious without any further processing of virion constituents.

Molecular basis for interference of defective interfering particles of pseudorabies virus with replication of standard virus

Serial passage of pseudorabies virus (PrV) at high multiplicity yields defective interfering particles (DIPs), but the sharp cyclical increases and decreases in titer of infectious virus that are observed upon continued passage at high multiplicity of most DIPs of other viruses are not observed with DIPs of PrV (T. Ben-Porat and A. S. Kaplan, Virology 72:471-479). We have studied the dynamics of the interactions of the virions present in a population of DIPs to assess the cis functions for which the genomes of the DIPs are enriched. The defective genomes present in one population of DIPs, [PrV(1)42], replicate preferentially over the nondefective genomes present in that virion population at early stages of infection, indicating that the DIP DNA is enriched for sequences that can serve as origins of replication at early stages of infection. This replicative advantage of the DIP DNA is transient and disappears at later stages of infection. The defective DNA does not appear to be encapsidated preferentially over the nondefective DNA present in this virion population, which might indicate that it is not enriched for cleavage-encapsidation sites. However, the nondefective DNA in the DIP virion population has become modified and has acquired reiterations of sequences originating from the end of the unique long (UL) region of the genome. Furthermore, both the infectious and defective genomes present in the DIP population compete for encapsidation more effectively than do the genomes of standard PrV. These results indicate that the defective genomes in the population of virions studied are enriched not only for an origin of replication but probably also for sequences necessary for efficient cleavage-encapsidation. Furthermore, the nondefective genomes present in this population of DIPs have also been modified and have acquired the ability to compete with the defective genomes for cleavage-encapsidation.

Induction of a new ribonucleotide reductase after infection of mouse L cells with pseudorabies virus

The mammalian ribonucleotide reductase consists of two nonidentical subunits, protein M1 and M2. M1 binds nucleoside triphosphate allosteric effectors, whereas M2 contains a tyrosine free radical essential for activity. The activity of ribonucleotide reductase increased 10-fold in extracts of mouse L cells 6 h after infection with pseudorabies virus. The new activity was not influenced by antibodies against subunit M1 of calf thymus ribonucleotide reductase, whereas the reductase activity in uninfected cells was completely neutralized. Furthermore, packed infected cells (but not mock-infected cells) showed an electron paramagnetic resonance spectrum of the tyrosine free radical of subunit M2 of the cellular ribonucleotide reductase. These data given conclusive evidence that on infection, herpesvirus induces a new or modified ribonucleotide reductase. The virus-induced enzyme showed the same sensitivity to inhibition by hydroxyurea as the cellular reductase. The allosteric regulation of the virus enzyme was completely different from the regulation of the cellular reductase. Thus, CDP reduction catalyzed by the virus enzyme showed no requirement for ATP as a positive effector, and no feedback inhibition was observed by dTTP or dATP. The virus reductase did not even bind to a dATP-Sepharose column which bound the cellular enzyme with high affinity.

Focus formation and neoplastic transformation by herpes simplex virus type 2 inactivated intracellularly by 5-bromo-2'-deoxyuridine and near UV light

The induction of focus formation in low serum and of neoplastic transformation of Syrian hamster embryo cells was examined after the expression of herpes simplex virus type 2 functions. Syrian hamster embryo cells infected at a high multiplicity (5 PFU/cell) with 5-bromo-2'-deoxyuridine-labeled herpes simplex virus type 2 (11% substitution of thymidine residues) were exposed to near UV light irradiation at various times postinfection. This procedure specifically inactivated the viral genome, while having little, if any, effect on the unlabeled cellular DNA. Focus formation in 1% serum and neoplastic transformation were observed in cells exposed to virus inactivated before infection, but the frequency was enhanced (15- to 27-fold) in cells in which the virus was inactivated at 4 to 8 h postinfection. Only 2 to 45 independently isolated foci were capable of establishing tumorigenic lines. The established lines exhibited phenotypic alterations characteristic of a transformed state, including reduced serum requirement, anchorage-independent growth, and tumorigenicity. They retained viral DNA sequences and, even at relatively late passage, expressed viral antigens, including ICP 10.

Interaction in replication between goose parvovirus strain B and duck plague herpesvirus

The goose parvovirus strain B was found to replicate only in cells with DNA synthesis. Cells in the stationary phase of DNA synthesis or maintained under 2 mM thymidine (TdR) in excess were refractive to the parvovirus infection. However, stationary phase cells could be rendered to produce progeny paroviruses by herpesvirus superinfection. In reverse, the replication of the herpesvirus was significantly inhibited by parvovirus preinfection of cells.