RNA polymerase activity and protein synthesis in brome mosaic virus-infected protoplasts

Viral protein synthesis in barley protoplasts inoculated with native and fractionated brome mosaic virus RNA

When barley protoplasts were inoculated with brome mosaic virus (BMV) RNAs 1 and 2, there was a pronounced synthesis of the 110,000- and 100,000-dalton virally coded proteins. In contrast, there was no detectable synthesis of any viral proteins following inoculation with RNA 3 alone or RNA 4 alone. When RNAs 1 and 2 were recombined with RNA 3 in the inoculum, the profile of proteins synthesized was identical to that following inoculation with similar quantities of unfractionated BMV RNA; i.e., the 35,000-dalton virally coded protein and coat protein were synthesized in addition to the two high-molecular-weight viral polypeptides. RNAs 1 and 2 were shown not to be selectively bound (in preference to RNAs 3 or 4); hence, these data reveal that one or both of these RNAs encode proteins involved in early events of infection, perhaps replication.

RNA-dependent RNA polymerase complex of Brome mosaic virus: analysis of the molecular structure with monoclonal antibodies

Viral RNA-dependent RNA polymerase (RdRp) plays crucial roles in the genomic replication and subgenomic transcription of Brome mosaic virus (BMV), a positive-stranded RNA plant virus. BMV RdRp is a complex of virus-encoded 1a and 2a proteins and some cellular factors, and associates with the endoplasmic reticulum at an infection-specific structure in the cytoplasm of host cells. In this study, we investigate the gross structure of the active BMV RdRp complex using monoclonal antibodies raised against the 1a and 2a proteins. Immunoprecipitation experiments showed that the intermediate region between the N-terminal methyltransferase-like domain and the C-terminal helicase-like domain of 1a protein, and the N terminus region of 2a protein are exposed on the surface of the solubilized RdRp complex. Inhibition assays for membrane-bound RdRp suggested that the intermediate region between the methyltransferase-like and the helicase-like domains of 1a protein is located at the border of the region buried within a membrane structure or with membrane-associated material.

The 3'-untranslated region of RNA1 as a primary determinant of temperature sensitivity of Red clover necrotic mosaic virus Canadian strain

Red clover necrotic mosaic virus Canadian strain (RCNMV-Can) induces symptoms on host plants at 17 degrees C, but not at 25 degrees C. We investigated the temperature sensitivity of RCNMV-Can in Nicotiana benthamiana plants and protoplasts using infectious transcripts of genomic RNAs 1 and 2. Viral RNAs accumulated in both inoculated and noninoculated leaves at 17 degrees C, whereas no viral RNAs were detected at 25 degrees C in either inoculated or noninoculated leaves. Similar temperature sensitivity in RNA accumulation was observed in protoplasts, and no viral RNAs were detected at temperatures above 22 degrees C. These results indicate that the temperature sensitivity of RCNMV-Can occurs at an early stage of infection, including during RNA replication. Using reassortant viruses and chimeric RNAs 1 between RCNMV-Can and the RCNMV Australian strain, which accumulates viral RNAs at nonpermissive temperatures for RCNMV-Can, we demonstrated that a viral determinant for the temperature sensitivity resides in the 3'-untranslated region of RNA1.

Small-Scale Isolation of Viral RNA-Dependent RNA Polymerase from Protoplasts Inoculated with In Vitro Transcripts

Cowpea chlorotic mottle virus (CCMV) replicated in tobacco suspension cell protoplasts inoculated with in vitro transcripts of CCMV RNA1, 2, and 3. CCMV RNA-dependent RNA polymerase (RdRp) isolated from these protoplasts specifically recognized CCMV and Brome mosaic virus (BMV) subgenomic RNA promoters and directed in vitro RNA synthesis in a manner indistinguishable from CCMV RdRp more laboriously isolated from systemically infected cowpea leaves. Omission of CCMV RNA3 from the protoplast inoculum or replacement with in vitro transcripts of BMV RNA3 reduced CCMV (+)-strand RNA1 and 2 accumulation to approximately 1/40 and approximately 1/10, respectively, of the level attained when CCMV RNA3 was present. The absence of CCMV RNA3 did not prevent assembly and isolation of highly active, template-dependent and template-specific CCMV RdRp, which directed synthesis of products identical in size to those of RdRp isolated from protoplasts inoculated with all three CCMV genomic RNAs. These results demonstrate that CCMV RNA1 and 2 are sufficient for CCMV replication and RdRp assembly in tobacco protoplasts. This approach for isolation of functional viral RdRp will be especially useful for viruses for which large quantities of infected tissue are unavailable, such as those with specific tissue tropisms or mutants incapable of systemic movement.

Brome mosaic virus defective RNAs generated during infection of barley plants

Brome mosaic virus (BMV) purified from systemically infected barley leaves 8 weeks post-inoculation (p.i.) contained defective RNAs (D-RNAs). The D-RNAs were detected in total and virion RNAs extracted from infected plants at 8 weeks p.i. or later, but not before, when barley plants had been inoculated with virions either containing or lacking D-RNA. The D-RNAs were derived from genomic RNA3 by double or mainly single deletions in the 3a protein ORF, and formed a heterogeneous population. By using in vitro transcripts of D-RNA synthesized from full-length cDNA clones, the D-RNAs were shown to replicate in a helper virus-dependent manner and to be packaged into virions in barley protoplasts. Subgenomic RNA4 was produced from the D-RNA and the coat protein was also expressed. Existence of the D-RNAs together with BMV genomic RNAs in inoculated protoplasts decreased the accumulation of 3a protein but it had no apparent effect on the accumulation of BMV genomic RNA3 or the coat protein. This is the first report of naturally occurring D-RNAs generated during prolonged infection with BMV.

Interaction of spermidine with viral RNA and its influence on protein synthesis

Addition of spermidine to a cell-free protein synthesizing system from wheat germ programmed with total brome mosaic virus (BMV) RNA resulted in a several-fold stimulation of amino acid incorporation. Increasing the spermidine concentration in the system led to inhibition of the overall protein synthesis, but the production of longer polypeptides was inhibited much more than that of the coat protein (shorter product). Analysis of the products synthesized under direction of BMV RNA 3 (longer product) and RNA 4 (coat protein) revealed that optimal translation of RNA 3 occurred at a much lower concentration of spermidine than that of RNA 4. Binding experiments with radioactive spermidine and BMV RNAs showed that the saturation of spermidine binding is achieved at a lower concentration of spermidine for RNA 3 than for RNA 4, which may suggest that the structure of RNA 4 is more compact than that of RNA 3. Taking into account the binding obtained at a spermidine concentration corresponding to optimal conditions of protein synthesis, it may be concluded that the optimum translation of these two mRNAs occurs when there is a similar level of RNA charge neutralisation, which implies a similar level of RNA structure stabilisation.