Partial reconstitution of tobacco mosaic virus

Dynamic DNA-controlled "stop-and-go" assembly of well-defined protein domains on RNA-scaffolded TMV-like nanotubes

A DNA-based approach allows external control over the self-assembly process of tobacco mosaic virus (TMV)-like ribonucleoprotein nanotubes: their growth from viral coat protein (CP) subunits on five distinct RNA scaffolds containing the TMV origin of assembly (OAs) could be temporarily blocked by a stopper DNA oligomer hybridized downstream (3') of the OAs. At two upstream (5') sites tested, simple hybridization was not sufficient for stable stalling, which correlates with previous findings on a non-symmetric assembly of TMV. The growth of DNA-arrested particles could be restarted efficiently by displacement of the stopper via its toehold by using a release DNA oligomer, even after storage for twelve days. This novel strategy for growing proteinaceous tubes under tight kinetic and spatial control combines RNA guidance and its site-specific but reversible interruption by DNA blocking elements. As three of the RNA scaffolds contained long heterologous non-TMV sequence portions that included the stopping sites, this method is applicable to all RNAs amenable to TMV CP encapsidation, albeit with variable efficiency most likely depending on the scaffolds' secondary structures. The use of two distinct, selectively addressable CP variants during the serial assembly stages finally enabled an externally configured fabrication of nanotubes with highly defined subdomains. The "stop-and-go" strategy thus might pave the way towards production routines of TMV-like particles with variable aspect ratios from a single RNA scaffold, and of nanotubes with two or even more adjacent protein domains of tightly pre-defined lengths.

Assembly of trans-encapsidated recombinant viral vectors engineered from Tobacco mosaic virus and Semliki Forest virus and their evaluation as immunogens

RNA virus vectors are attractive vaccine delivery agents capable of directing high-level gene expression without integration into host cell DNA. However, delivery of non-encapsidated RNA viral vectors into animal cells is relatively inefficient. By introducing the tobacco mosaic virus (TMV) origin of assembly into the RNA genome of Semliki Forest virus (SFV), we generated an SFV expression vector that could be efficiently packaged (trans-encapsidated) in vitro by purified TMV coat protein (CP). Using cellular assays, pseudovirus disassembly, RNA replication and reporter gene expression were demonstrated. We also evaluated the immune response to trans-encapsidated recombinant SFV carrying a model antigen gene (beta-galactosidase) in C57/B6 mice. Relative to RNA alone, vector encapsidation significantly improved the humoral and cellular immune responses. Furthermore, reassembly with recombinant TMV CPs permitted the display of peptide epitopes on the capsid surface as either genetic fusions or through chemical conjugation, to complement the immunoreactivity of the encapsidated RNA genetic payload. The SFV vector/TMV CP system described provides an alternative nucleic acid delivery mechanism that is safe, easy to manufacture in vitro and that also facilitates the generation of unique nucleic acid/protein antigen compositions.

Sequence from the assembly nucleation region of TMV RNA

In an effort to isolate RNA sequences containing the assembly nucleation region, uniformly 32P-labeled tobacco mosaic virus RNA was partially digested with pancreatic ribonuclease, and the mixture of fragments was incubated with limited amounts of tobacco mosaic virus protein disks in conditions favorable for reconstitution. The RNA fragments which became encapsidated were purified and sequenced by conventional techniques. The sequence of the first 139 nucleotides of P1, the principal encapsidated fragment, is AGGUUUGAGAGAGAAGAUUACAAGCGUGAGAGACGGAGGGCCCAUGGAACUUACAGAAGAAGUUGUUGAUGAGUUCAUGGAAGAUGUCCCUAUGUCAAUCAGACUUGCAAAGUUUCGAUCUCGAACCGGAAAAAAGAGU. Residues 1--110 of P1 overlap the assembly origin isolated and characterized in the accompanying papers by Zimmern (1977) and Zimmern and Butler (1977). Our results, taken in conjunction with the two accompanying papers, define the sequence of much of the nucleation region as well as sequences flanking it on both sides. The features of the P1 sequence which may have role in the nucleation reaction are discussed in detail in the text.

Configuration of tobacco mosaic virus, RNA during virus assembly

When TMV reassembles, the uncoated RNA is folded back along the growing rod, probably down the central hole. This surprising configuration is essential for rapid elongation--presumably supplying RNA to its site of incorporation while keeping the bulk of the free RNA out of the way.

Location of the cistron of the tobacco mosaic virus coat protein

Treatment of tobacco mosaic virus (TMV) RNA with T1 RNase under mild conditions cuts the RNA molecule into a large number of fragments, only a few of which may be specifically recognized by disks of TMV protein. It has been shown elsewhere that these specifically recognized RNA fragments are a part of the coat protein cistron, the portion coding for amino acids 95 to 129 of the coat protein. It is reported that different size classes of partially uncoated virus particles were prepared by limited reconstitution between TMV RNA and protein or by partial stripping of intact virus with DMSO. Both procedures produce nucleoprotein rods in which the 5'-terminal portion of the RNA is encapsidated and the 3'-terminal region is free. The free and the encapsidated portions of the RNA were each tested for the ability to give rise to the aforesaid specifically recognized fragments of the coat protein cistron upon partial T1 RNase digestion. It was found that only the 3'-terminal third of the virus particle need to be uncoated in order to expose the portion of the RNA molecule from which these fragments are derived. We conclude, therefore, that the coat protein cistron is situated upon the 3'-terminal third of the RNA chain, i.e. within 2000 nucleotides of the 3'-end.

Some characters of viruses reconstituted with components from different tobacco mosaic virus strains

Mixed reconstitution was performed using the components of tobacco mosaic virus‐ordinary strain (TMV‐OM) and bean strain (TMV‐B) under conditions of 0.1 m pH 7.3 sodium pyrophosphate at 30 C for 6–24 hr. The recoveries of the reconstituted viruses, TMV‐OM‐RNA+TMV‐B‐protein and TMV‐OM‐protein+TMV‐B‐RNA, were 1.5–26.6% and both the viruses were infectious. Sucrose density gradient centrifugation of these 2 reconstituted viruses showed 2 peaks, both of which were infectious. The bottom peak sedimented near the position of the standard TMV, and its ratio of O.D.260mμ/O.D.280mμ was 1.22–1.29. The particles in this peak had the same length as that of the standard TMV. The top peak had the ratio of 1.28–1.41, and the length of particles in this peak were shorter than the standard TMV. The pattern of ECTEOLA‐cellulose column chromatography of the reconstituted virus was similar to that of standard TMV. Further, it was confirmed that the biological characters of the reconstituted particles were the same as those of the RNA‐donor TMV.