Bromovirus RNA Replication and Host Specificity

Positive strand RNA viruses, i.e., viruses that encapsidate single stranded messenger-sense RNA, are a numerically and practically important class of viruses in eukaryotic hosts. Such viruses are particularly significant and well diversified in plants, comprising by far the largest single class of known plant viruses (Matthews, 1982). Research interest in positive strand RNA viruses derives from many perspectives, including their role as highly successful pathogens, their facility at high level gene transmission and expression, and the novelty of their RNA-dependent replication, gene expression, and recombination pathways. The value of understanding RNA-based genetic processes is underscored by the growing suggestion that RNA genomes may have preceded DNA genomes as one stage in the early evolution of life (Joyce, 1989). Thus, as well as illuminating their own unusual biochemistry and genetics, RNA virus studies may provide general insights into replication, survival, and evolution in the putative primordial RNA world. In addition, since many of its features are widely conserved among otherwise diverse viruses of both plants and animals (see below), the viral RNA replication apparatus is a potential target for the development of broad spectrum antiviral agents, which would have great practical benefits. The genetic pathways of RNA viruses have also provided and should continue to provide further new ways to amplify and regulate gene expression in applied genetic engineering (French et al., 1986; Xiong et al., 1989). Finally, viruses possess a number of attractive features as general models for plant-microbe interactions, including uniquely small genomes, ready manipulation by recombinant DNA techniques, and a growing understanding of their life cycle. This chapter discusses some aspects of replication, recombination, and host specificity in one representative group of positive strand RNA plant viruses, the bromoviruses.