Effect of 2-thiouracil on RNA and protein synthesis in synchronous and asynchronous infections of tobacco mosaic virus

Lethal mutagenesis of an RNA plant virus via lethal defection

Lethal mutagenesis is an antiviral therapy that relies on increasing the viral mutation rate with mutagenic nucleoside or base analogues. Currently, the molecular mechanisms that lead to virus extinction through enhanced mutagenesis are not fully understood. Increasing experimental evidence supports the lethal defection model of lethal mutagenesis of RNA viruses, where replication-competent-defectors drive infective virus towards extinction. Here, we address lethal mutagenesis in vivo using 5-fluorouracil (5-FU) during the establishment of tobacco mosaic virus (TMV) systemic infections in N. tabacum. The results show that 5-FU decreased the infectivity of TMV without affecting its viral load. Analysis of molecular clones spanning two genomic regions showed an increase of the FU-related base transitions A → G and U → C. Although the mutation frequency or the number of mutations per molecule did not increase, the complexity of the mutant spectra and the distribution of the mutations were altered. Overall, our results suggest that 5-FU antiviral effect on TMV is associated with the perturbation of the mutation-selection balance in the genomic region of the RNA-dependent RNA polymerase (RdRp). Our work supports the lethal defection model for lethal mutagenesis in vivo in a plant RNA virus and opens the way to study lethal mutagens in plant-virus systems.

Anti-herpesvirus activities and cytotoxicities of 2-thiopyrimidine nucleoside analogues in vitro

Twenty 2-thiopyrimidine nucleoside analogues were synthesized and examined for inhibitory activity against herpes simplex virus (HSV) type 1 and 2, varicella-zoster virus (VZV), human cytomegalovirus (HCMV) and thymidine kinase-deficient HSV (HSV-TK-) replication in vitro. 2-thiouracil (thymine) arabinoside, 2'-deoxy-2-thiouridine (or 2-thiothymidine) and their 5-halogenated derivatives showed anti-HSV activity in both RPM18226 (human B-lymphoblastoid cells) and MRC-5 (human embryo lung cells). 2'-deoxy-5-halogenated-2-thiocytidines were also inhibitory against HSV, whereas 2-thiocytosine arabinoside and its derivatives were not inhibitory against HSV replication, except 5-bromo and 5-iodo congeners (TN-31, TN-32). Substitution of the halogen atom at the 5-position of the pyrimidine rings to an atom with a higher molecular weight increased anti-HSV and VZV activities, except for the anti-HSV activity of 2-thiouracil arabinosides. 2'-deoxy-5-methyl-, and 2'-deoxy-5-iodo-2-thiouridines (TN-17, TN-44) showed the most potent anti-HSV activity, and 2'-deoxy-5-chloro- and 2'-deoxy-5-bromo-2-thiocytidines were potent inhibitors of VZV replication. However, none of the compounds inhibited HCMV and HSV-TK- replication. TN-31 and TN-32 were shown to inhibited HCMV and HSV-TK- as well as HSV and VZV replication. The cytotoxicity of the 2-thio-pyrimidine nucleoside analogues was less than that of the 2-oxy-congeners of the compounds (5-iodo-2'-deoxyuridine, 5-iodo-2'-deoxycytidine, thymine arabinoside and cytosine arabinoside). The selectivity index of 2'-deoxy-5-iodo-2-thiouridine (TN-44) was higher than that of 5-iodo-deoxyuridine. TN-17 and TN-44 were not cytotoxic to resting or stimulated human peripheral blood mononuclear cells at 400 microM, although TN-32 was cytotoxic at a concentration of 20 microM.

Regulation of tobamovirus gene expression

This chapter discusses tobacco mosaic virus (TMV) strains U1, OM, L, CGMMV, 0, and Cc. The production of each TMV protein is regulated differently, both in amounts and times of production. The chapter discusses some of the strategies that tobamoviruses uses to control gene expression: (1) different subgenomic RNA promoter/leader sequences control timing of expression of genes, (2) genes expressed via subgenomic mRNAs are expressed in decreasing amounts with increasing distances from the 3' terminus, and (3) TMV mRNAs appear to be translationally regulated differently from host mRNAs. Genome organization affects gene expression, but it appears to be equally important for the efficiency of replication and the ability of the genomic structure to be stably propagated. Different virus groups have evolved different gene arrangements. Tobamovirus genes expressed via subgenomic mRNAs appear to be expressed in increasing amounts when positioned nearer the 3’ terminus.