Conditional lethal mutants of animal viruses

Methods for the Manipulation of Herpesvirus Genome and the Application to Marek's Disease Virus Research

Herpesviruses are a group of double-strand DNA viruses that infect a wide range of hosts, including humans and animals. In the past decades, numerous methods have been developed to manipulate herpesviruses genomes, from the introduction of random mutations to specific genome editing. The development of genome manipulation methods has largely advanced the study of viral genes function, contributing not only to the understanding of herpesvirus biology and pathogenesis, but also the generation of novel vaccines and therapies to control and treat diseases. In this review, we summarize the major methods of herpesvirus genome manipulation with emphasis in their application to Marek’s disease virus research.

A Temperature-Sensitive Lesion in the N-Terminal Domain of the Rotavirus Polymerase Affects Its Intracellular Localization and Enzymatic Activity

Temperature-sensitive (ts) mutants of simian rotavirus (RV) strain SA11 have been previously created to investigate the functions of viral proteins during replication. One mutant, SA11-tsC, has a mutation that maps to the gene encoding the VP1 polymerase and shows diminished growth and RNA synthesis at 39°C compared to that at 31°C. In the present study, we sequenced all 11 genes of SA11-tsC, confirming the presence of an L138P mutation in the VP1 N-terminal domain and identifying 52 additional mutations in four other viral proteins (VP4, VP7, NSP1, and NSP2). To investigate whether the L138P mutation induces a ts phenotype in VP1 outside the SA11-tsC genetic context, we employed ectopic expression systems. Specifically, we tested whether the L138P mutation affects the ability of VP1 to localize to viroplasms, which are the sites of RV RNA synthesis, by expressing the mutant form as a green fluorescent protein (GFP) fusion protein (VP1_L138P-GFP) (i) in wild-type SA11-infected cells or (ii) in uninfected cells along with viroplasm-forming proteins NSP2 and NSP5. We found that VP1_L138P-GFP localized to viroplasms and interacted with NSP2 and/or NSP5 at 31°C but not at 39°C. Next, we tested the enzymatic activity of a recombinant mutant polymerase (rVP1_L138P) in vitro and found that it synthesized less RNA at 39°C than at 31°C, as well as less RNA than the control at all temperatures. Together, these results provide a mechanistic basis for the ts phenotype of SA11-tsC and raise important questions about the role of leucine 138 in supporting key protein interactions and the catalytic function of the VP1 polymerase.IMPORTANCE RVs cause diarrhea in the young of many animal species, including humans. Despite their medical and economic importance, gaps in knowledge exist about how these viruses replicate inside host cells. Previously, a mutant simian RV (SA11-tsC) that replicates worse at higher temperatures was identified. This virus has an amino acid mutation in VP1, which is the enzyme responsible for copying the viral RNA genome. The mutation is located in a poorly understood region of the polymerase called the N-terminal domain. In this study, we determined that the mutation reduces the ability of VP1 to properly localize within infected cells at high temperatures, as well as reduced the ability of the enzyme to copy viral RNA in a test tube. The results of this study explain the temperature sensitivity of SA11-tsC and shed new light on functional protein-protein interaction sites of VP1.

Instability and reiteration of DNA sequences within the vaccinia virus genome

The sequence arrangement within the nontranscribed portion of the inverted terminal repetition of the vaccinia virus genome exists in quasi-stable and unstable forms that are not distinguishable on the basis of viral infectivity. The unstable forms, which composed about 20% of a serially passaged stock of virus, were recognized by terminal heterogeneity on restriction endonuclease analysis. Instead of a single terminal fragment from each end of the genome, an array of eight or more fragments differing in size by 1650-base-pair increments was detected. This feature was not eliminated by repeated plaque purification, indicating that the population of DNA molecules with various numbers of reiterations can rapidly evolve from the DNA of a single virus particle. However, at each successive round of plaque purification, about 20% of the unstable isolates revert back to the more stable form. Stable forms are characterized by the presence of a set of 13-17 tandem 70-base-pair repeats on each side of a 435-base-pair intervening sequence near both ends of the genome. In contrast, the unstable forms possess sets of tandem repeats and intervening sequences that alternate many times in series. The transition between the two genomic forms and the evolution of the unstable form appear to be mediated by recombinational events.

A spontaneous temperature sensitive mutant of Japanese encephalitis virus: preliminary characterization

A spontaneously arising temperature sensitive (ts) mutant of Japanese encephalitis virus (JEV), ts104, was isolated from chick fibroblast (CF) cell cultures of JEV strain M 1/311. Strain ts104 was plaque purified and characterized to ascertain its potential as a candidate for a live vaccine. Parameters of its growth, temperature lability, immunogenicity and virulence were examined. Ts104 has been shown to be stable ts JEV strain, multiplying as well as the parent strain in CF cultures at 35 degrees C, but not mutiplying at 39 degrees C. It was avirulent for embryonated chicken eggs incubated at 39 degrees C and of reduced virulence for intracerebrally (i.c.) inoculated mice as measured by LD50 in weanling mice and average day of death in weanling and suckling mice. Intraperitoneal injection of adult mice with either parent or ts strain resulted in similar levels of protection against challenge with either strain. The potential of ts104 as a candidate live JEV vaccine strain is discussed.

Temperature-sensitive mutants of murine leukemia virus. IV. Further physiological characterization and evidence for genetic recombination

Several temperature-sensitive mutants of the Rauscher strain of murine leukemia virus representing three distinct physiological groups have been further characterized. Genetic recombination between different pairs of these mutants has been demonstrated. Several representative genetic recombinants were isolated and shown to replicate equally well at the permissive (31 C) and nonpermissive (38 C) temperatures and to have serological characteristics of the wild-type parental virus. Alternative models for the mechanisms involved in recombination between type C RNA viruses are discussed.

Method for isolation and selection of temperature-sensitive mutants of herpes simplex virus

A method for induction and selection of temperature-sensitive mutants of herpes simplex virus is presented. After the infected cells were treated with 5-bromodeoxyuridine, the virus was extracted by repeated freezing and thawing and cloned in microcultures which were then incubated at permissive temperature until viral plaques appeared. The microcultures were then replicated at nonpermissive temperature. Clones not forming plaques in these latter were further purified and examined for temperature-sensitive characteristics. Viral clones mutated in plaque-forming ability or in yield were obtained and preliminarily characterized.

Isolation and characterization of Sendai virus temperature-sensitive mutants

Ten temperature-sensitive mutants of Sendai virus, a paramyxovirus, were isolated and partially characterized. The mutants replicated in chicken embryo lung cells at 30 C, but not at 38 C; wild-type virus grew equally well at both temperatures. Complementation tests divided the mutants into seven groups. Six groups synthesized neither infectious virus nor RNA when incubated at 38 C from the beginning of infection. Temperature shift-up experiments demonstrated that three of these complementation groups were blocked in early steps required for RNA synthesis, but these gene functions were not needed throughout the replicative cycle. In contrast, the other three RNA-negative complementation groups were defective throughout the replicative cycle in functions required for virus-specific RNA synthesis. Only one mutant, which complemented all of the above, synthesized RNA but not infectious virus when placed at 38 C; the hemagglutinin of this mutant functioned only at the permissive temperature.

Selection and preliminary characterization of temperature-sensitive mutants of type 5 adenovirus

Eight temperature-sensitive (ts) mutants that replicate normally at 32 C but poorly, if at all, at 39.5 C have been isolated from mutagenized stocks of a wild-type strain of type 5 adenovirus. Three mutagens were employed: nitrous acid, hydroxylamine, and nitrosoguanidine. Ts mutants were isolated from mutagenized viral stocks with frequencies between 0.01 and 0.1%. All eight mutants had reversion frequencies of 10(-5) or less. Complementation experiments in doubly infected cultures at the nonpermissive temperature separated the mutants into three nonoverlapping complementation groups. Complementation yields ranged from a 2.3- to a 3,000-fold increase over the sums of the yields from the two singly infected controls. Genetic recombination was also demonstrated; approximate recombination frequencies ranged from 0.1 to 15%. Preliminary biochemical and immunological characterization of the mutants indicated that: (i) the single mutant in complementation group I did not replicate its deoxyribonucleic acid (DNA) or synthesize late proteins at the nonpermissive temperature but did inhibit host DNA synthesis to 25% of an uninfected control; (ii) the four group II mutants replicated viral DNA, shut off host DNA synthesis, synthesized penton base and fiber, but did not synthesize immunologically detectable hexon; the three mutants in complementation group III synthesized viral DNA, shut off host DNA synthesis, and made immunologically reactive capsid proteins (hexon, penton base, and fiber).

Isolation of temperature-sensitive conditional lethal mutants of "fixed" rabies virus

In an attempt to induce temperature-sensitive (ts) conditional lethal mutants of rabies virus, stocks of a plaque-purified substrain of strain CVS fixed rabies virus were subjected to mutagenesis by HNO(2), 5-fluorouracil, or 5-azacytidine. It was necessary to prepare virus stocks from clones of mutagenized virus selected at random and to test subsequently each stock for possible ts characteristics by measuring its relative capacity for growth at permissive (33 C) and nonpermissive (40.5 C) temperatures. Five ts mutants were detected in tests of 161 clones of mutagenized virus. Each of the mutants exhibited a remarkably low incidence of reversion and little demonstrable "leakiness." One of the five ts mutants (ts2), which formed formed very small plaques, and another (ts1), which formed plaques of only slightly reduced size, were further characterized. Virus ts1 was more thermostable at 40.5 C than the parental virus, but the ts2 mutant was unchanged in this respect. The ts1 virus exhibited normal pathogenicity for mice, but ts2 virus caused a very irregular death pattern. Both deaths and survivors immune to rabies virus challenge were noted in all groups of mice inoculated with ts2 virus, regardless of the virus dose.