Plant RNA viruses

Two types of expression systems based on plant RNA viruses have been developed for production of immunogenic peptides and proteins in plants epitope presentation systems (short antigenic peptides fused to the CP that are displayed on the surface of assembled viral particles) and polypeptide expression systems (these systems express the whole unfused recombinant protein that accumulates within the plant). 

The known catalytic repertoire of ribozymes continues to expand. Some virusoids, small RNAs associated with plant RNA viruses, include a structure that promotes a self-cleavage reaction the hammerhead ribozyme illustrated in Figure 26-25 is in this class, catalyzing the hydrolysis of an internal phosphodiester bond. The splicing reaction that occurs in a spliceosome seems to rely on a catalytic center formed by the U2, U5, and U6 snRNAs (Fig. 26-16). And perhaps most important, an RNA component of ribosomes catalyzes the synthesis of proteins (Chapter 27). 

In addition to the catalytic action served by the snRNAs in the formation of mRNA, several other enzymatic functions have been attributed to RNA. Ribozymes are RNA molecules with catalytic activity. These generally involve transesterification reactions, and most are concerned with RNA metabofism (spfic-ing and endoribonuclease). Recently, a ribosomal RNA component was noted to hydrolyze an aminoacyl ester and thus to play a central role in peptide bond function (peptidyl transferases see Chapter 38). These observations, made in organelles from plants, yeast, viruses, and higher eukaryotic cells, show that RNA can act as an enzyme. This has revolutionized thinking about enzyme action and the origin of life itself. 

Similarly, the case for monophyly is hard to sustain for the origin of multicomponent (segmented) RNA viruses (positive single stranded RNA viruses, e.g. tobravirus, cucumovirus and bromovirus) and plant bipartite DNA viruses (single stranded DNA viruses, e.g. geminivirus) where coinfection ( two for one ) is required for the production of new virions (see Summary in Shostak, 1999). 

The majority of viruses that infect plants have single-stranded, positive-sense RNA genomes. It has therefore been necessary to use infectious cDNA clones for the in vitro manipulation of RNA viruses, allowing them to be developed as effective tools for the commercial production of target proteins in plants. This approach has also been used to study the genetic and metabolic profiles of both viruses and their host plants. Siegel [14] conceptualized the potential use of RNA viruses as expression vectors. Brome mosaic virus (BMV) and Tobacco mosaic vims (TMV) were the first two RNA viruses to be converted into expression vectors. These vectors have since been pro-... 

The tobacco mosaic virus (center right), a plant pathogen, has a structure similar to that of MB, but contains ssRNA instead of DNA. The poliovirus, which causes poliomyelitis, is also an RNA virus. In the influenza virus, the pathogen that causes viral flu, the nucleocapsid is additionally surrounded by a coat derived from the plasma membrane of the host cell (C). The coat carries viral proteins that are involved in the infection process. 

Voinnet, O., Pinto, Y.M., and Baulcombe, D.C. (1999). Suppression of gene silencing a general strategy used by diverse DNA and RNA viruses of plants. Proc. Natl. Acad. Sci. U.S.A. 96 14147-14152. 

DNA provirus form of the RNA virus that causes AIDS15 and for bacterial viruses such as T 7 bacteriophage (39,936 bp).16 Also determined in the 1980s were sequences of human mitochondrial DNA (16,598 bp),17 and of chloroplast DNA from the tobacco plant (155,844 bp).18... 

A large number of icosahedral RNA viruses of diameter 28-30 nm (Fig. 7-14) attack plants, causing diseases such as tomato bushy stunt,504 southern bean mosaic,505 or turnip yellow mosaic. Best known of the helical RNA viruses is the tobacco mosaic virus (Figs. 5-41,7-8).506 507a Its genome contains 6395 nucleotides as linear ssRNA. Many strains are known. Related viruses cause cucumber green mottle508 and other plant diseases. 

Genes in all cellular organisms are made of DNA. The same is true for some viruses, but for others the genetic material is RNA. Viruses are genetic elements enclosed in protein coats that can move from one cell to another but are not capable of independent growth. One well-studied example of an RNA virus is the tobacco mosaic virus, which infects the leaves of tobacco plants. This virus consists of a single strand of RNA (6930 nucleotides) surrounded by a protein coat of 2130 identical subunits. An RNA-directed RNA polymerase catalyzes the replication of this viral RNA. 

One of the early plant viruses used was the cauliflower mosaic virus (CaMV), a double stranded DNA virus that replicates through an RNA intermediate [38]. Using this virus extra DNA insertions are difficult to achieve, and often molecular recombinations rapidly eliminate the inserted transgene. In addition, replication of this virus requires an RNA step which can introduce errors because the inverse transcriptase does not have a proof-reading activity. Similarly, regarding RNA viruses, two viruses the tobacco mosaic virus (TMV) and the cowpea mosaic virus (CMV) have been successfully employed [39]. 

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