Genome Expression of Plant Positive-strand RNA Viruses

Introduction. Plant viruses have played an important role in the development of techniques for the study and understanding of virus structure, protein structure and the nature and expression of genetic information. Tobacco mosaic virus (TMV), for example, was the first virus to be purified and the first virus whose nucleic acid was shown to be the infective component. TMV RNA was the first natural messenger used to ‘programme’ protein synthesis in vitro, albeit in Escherichia coli extracts which we now know, in retrospect, were not synthesizing proteins with fidelity. It was, however, an important step towards the now widely used method to demonstrate what genetic information is encoded in virus RNA, namely in vitro translation. The development of plant protoplast systems has added another tool for studying the expression of plant virus genes. These approaches have in recent years also been applied to several plant viruses which, unlike TMV or turnip yellow mosaic virus (TYMV), have encapsidated split genomes with genetic information divided among two or three RNA components.