Temperature-sensitive mutants of Chandipura virus. I. Inter- and intragroup complementation

Leader RNA binding ability of Chandipura virus P protein is regulated by its phosphorylation status: a possible role in genome transcription-replication switch

The molecular events associated with the transcriptive and replicative cycle of negative-stranded RNA viruses are still an enigma. We took Chandipura virus, a member of the Rhabdoviridae family, as our model system to demonstrate that Phosphoprotein P, besides Nucleocapsid protein N, also acts as a leader RNA-binding protein in its unphosphorylated form, whereas CKII-mediated phosphorylation totally abrogates its RNA-binding ability. However, interaction between P protein and leader RNA can be distinguished from N-mediated encapsidation of viral sequences. Furthermore, P protein bound to leader chain can successively recruit N protein on RNA while itself being replaced. We also observed that the accumulation of phosphorylation null mutant of P protein in cells results in enhanced genome RNA replication with concurrent increase in the viral yield. All these results led us to propose a model explaining viral transcription-replication switch where Phosphoprotein P acts as a modulator of genome transcription and replication by its ability to bind to the nascent leader RNA in its unphosphorylated form, promoting read-through of the transcription termination signals and initiating nucleocapsid assembly on the nascent RNA chain.

Single serine phosphorylation within the acidic domain of Chandipura virus P protein regulates the transcription in vitro

Bacterially expressed unphosphorylated P protein of Chandipura Virus was found to be efficiently phosphorylated in vitro by casein kinase II (CKII). The phosphorylated form of the P protein supported the transcription in vitro but the unphosphorylated form could not. Kinetic data suggests that CKII incorporates one molecule of phosphate. Western blotting with monoclonal antibody against phosphoserine and phosphoaminoacid analysis confirmed that the phosphate accepting residue was serine. Comparison with P protein of other viruses and tryptic digest of the phosphorylated protein predicted the ser62 was the probable site for phosphorylation. This was further confirmed by substituating ser62 with alanine by site-directed mutagenesis. CKII was unable to phosphorylate the mutated P protein which in turn could not support the transcription in vitro. The phosphorylated P protein eluted from the gel filtration at the position of its dimer in contrast to the unphosphorylated protein which eluted as monomer.

Primary structure of the vesicular stomatitis virus polymerase (L) gene: evidence for a high frequency of mutations

A consensus sequence of the polymerase (L) gene of vesicular stomatitis virus, derived from three genomic cDNA copies, is presented. This analysis completes the primary structure of the vesicular stomatitis virus genome, totaling 11,162 bases. The L gene alone spans 6,380 nucleotides and codes for a basic 2,109-amino-acid protein with a molecular weight of 241,012. Sixteen point mutations were detected among cDNA clones prepared from viral RNA of the same strain, representing direct evidence for either the high mutability of vesicular stomatitis virus, the infidelity of reverse transcription during cDNA synthesis, or a combination of both. Some mutation, if present in the viral genome, would result in the translation of incomplete L proteins. For example, two out of four cDNA copies which covered the same region of the L gene had a single-base deletion in the exact same position, whereas the other two clones did not, strongly suggesting that a subpopulation of the genomic RNA may contain this lethal mutation. These lethal mutants define a new class of defective and most likely interfering particles which are indistinguishable in size from the parental virus and can be distinguished only by direct sequencing. We suggest that because of its infidelity, the viral polymerase itself introduces mutations and because of its size, most of these mutations are localized within the polymerase gene. In persistently infected cells in which the selective pressures on the polymerase are different, some of these L gene mutations may further erode the accuracy of the polymerase and thereby lead to the increased mutation rate that is characteristic of this type of infection.

Nongenetic complementation in VSV: asymmetric contribution of the L proteins of each parent in the rescue of group I ts mutants

Complementation group I temperature-sensitive mutants of vesicular stomatitis virus (VSV) are rescued at nonpermissive temperature by UV-irradiated virus. Rescue is a nongenetic process mediated by the structural L protein molecules of the parental irradiated virus. This is shown by action spectra analysis (V. Deutsch, B. Muel, and G. Brun (1977), Virology 77, 294-305), efficiency of rescue at doses high enough to inactivate every gene I, and rescue by irradiated defective-interfering short particles. Viral molecular synthesis at 39.6 degrees using the ts genomes as templates and stimulated by UV-irradiated virus is shown by gel electrophoresis of rescued virion proteins. Parental strain ts 053(I) is thermolabile in vitro while the thermostability of the rescued virions, genetically characterized as ts group I virus, is identical to that of helper wt or ts+ revertant. This suggests that some parental Lwt molecules are reincorporated in the rescued virions, together with newly synthesized Lts molecules. Efficiency variation of the rescue as a function of the multiplicity of infection of the UV-irradiated virions depends on the m.o.i. of the unirradiated ts I mutants. This result suggests that rescue depends on the concentration of the helper L molecules and also of that of the L initially bound to the ts template. That L of both parents contribute to rescue is supported by the observations that (1) rescue by UV-wt is strongly diminished after in vitro heating of thermolabile ts O53(I). (2) Intragenic rescue can be demonstrated: The helper activity of a given UV-ts I mutant is different according to the unirradiated ts I mutant used; the activity of helper L associated with different templates in intragenic heterologous combinations is higher or lower than its activity in the homologous combination (self-rescue control), instead of being equal as expected. (3) Efficiency of rescue by UV-wt also varies according to the ts I mutant. The initially bound Lts seems therefore to play a role important and different from that of the helper L. Contribution of both parent L to rescue seems thus to be qualitatively different.

Experimental models of virus-induced demyelination of the central nervous system

One of the arguments in favor of a viral pathogenesis for multiple sclerosis is the existence of several experimental and natural animal models of virus-induced primary demyelination. This review deals comprehensively with such models. Well-known examples of demyelinating viral infections in their natural host are JHM, Theiler, visna, and canine distemper encephalomyelitides. Recent reports of experimental murine infections with pathogens such as vesicular stomatitis, Chandipura, herpes simplex, Venezuelan equine encephalomyelitis, and Semliki Forest viruses are also discussed. The thrust of the review is to include viral models suspected of producing primary demyelination on an immunopathological basis.

Enhanced mutability associated with a temperature-sensitive mutant of vesicular stomatitis virus

Temperature-sensitive (ts) mutant tsD1 of vesicular stomatitis virus, New Jersey serotype, is the sole representative of complementation group D. Clones derived from this mutant exhibited three different phenotypes with respect to electrophoretic mobility of the G and N polypeptides of the virion in sodium dodecyl sulfate-polyacrylamide gel. Analysis of non-ts pseudorevertants showed that none of the three phenotypes was associated with the temperature sensitivity of mutant tsD1. Additional phenotypes, some also involving the NS polypeptide, appeared during sequential cloning, indicating that mutations were generated at high frequency during replication of tsD1. Furthermore, mutations altering the electrophoretic mobility of the G, N, NS, and M polypeptides were induced in heterologous viruses multiplying in the same cells as tsD1. These heterologous viruses included another complementing ts mutant of vesicular stomatitis virus New Jersey and ts mutants of vesicular stomatitis virus Indiana and Chandipura virus. Complete or incomplete virions of tsD1 appeared to be equally efficient inducers of mutations in heterologous viruses. Analysis of the progeny of a mixed infection of two complementing ts mutants of vesicular stomatitis virus New Jersey with electrophoretically distinguishable G, N, NS, and M proteins yielded no recombinants and excluded recombination as a factor in the generation of the electrophoretic mobility variants. In vitro translation of total cytoplasmic RNA from BHK cells indicated that post-translational processing was not responsible for the aberrant electrophoretic mobility of the N, NS, and M protein mutants. Aberrant glycosylation could account for three of four G protein mutants, however. Some clones of tsD1 had an N polypeptide which migrated faster in sodium dodecyl sulfate-polyacrylamide gel than did the wild type, suggesting that the polypeptide might be shorter by about 10 amino acids. Determination of the nucleotide sequence to about 200 residues from each terminus of the N gene of one of these clones, a revertant, and the wild-type parent revealed no changes compatible with synthesis of a shorter polypeptide by premature termination or late initiation of translation. The sequence data indicated, however, that the N-protein mutant and its revertant differed from the parental wild type in two of the 399 nucleotides determined. These sequencing results and the phenomenon of enhanced mutability associated with mutant tsD1 reveal that rapid and extensive evolution of the viral genome can occur during the course of normal cytolytic infection of cultured cells.

Temperature-sensitive mutants of Chandipura virus. II. Phenotypic characteristics of the six complementation groups

Fifty temperature-sensitive (ts) mutants of the rhabdovirus Chandipura virus have been classified into six complementation groups designated ChI to ChVI. Group ChI contains 44 mutants, group ChII contains 2 mutants, and the remaining groups have 1 mutant each. Mutants in groups ChI, ChIII, ChIV, and ChVI had RNA-negative phenotypes in experiments measuring amplification of RNA synthesis at restrictive temperature. The two mutants in group ChII had RNA-positive phenotypes, and the virions were thermolabile. Mutant ts Ch851 of group ChV was also RNA positive, and the M polypeptide of this mutant appeared to be unstable in cells incubated at restrictive temperature. It is likely, therefore, that complementation groups ChII and ChV represent the genes coding for the two viral proteins of the virion envelope. No precise assignment can be made in the case of the four RNA-negative groups, since all the mutants examined showed some polymerase activity in vitro at restrictive temperature. An attempt to obtain polymerase mutants by screening for sensitivity to rifampin was not successful. Six temperature-dependent host range mutants (the tdCE phenotype) of Chandipura virus failed to multiply in chicken embryo cells at restrictive temperature, but otherwise they differed in their host range properties from similar mutants of vesicular stomatitis virus.