Advances in the molecular biology of tombusviruses: gene expression, genome replication, and recombination.

The tombusviruses are among the most extensively studied messenger-sensed RNA plant viruses. Over the past decade, there have been numerous important advances in our understanding of the molecular biology of members in this genus. Unlike most other RNA viruses, the synthesis of tombusvirus proteins has been found to involve an atypical translational mechanism related to the uncapped and nonpolyadenylated nature of their genomes. Tombusviruses also appear to employ an unusual mechanism for transcription of the sg mRNAs that template translation of a subset of their viral proteins. In addition to these new insights into tombusvirus gene expression, there has also been significant progress made in our understanding of tombusvirus RNA replication. These studies have been facilitated greatly by small genome-derived RNA replicons, referred to as defective interfering RNAs. In addition, the development of an in vitro system to study viral RNA synthesis has allowed for dissection of some of the steps involved in the replication process. Another exciting recent advance has been the creation of yeast-based systems that support amplification of tombusvirus RNA replicons and will allow the identification of host factors involved in viral RNA synthesis. Lastly, the recombinogenic nature of tombusvirus genomes has made them ideal systems for studying RNA-RNA recombination and genetic rearrangements, both in vivo and in vitro. In this review, we compile recent information on each of the aforementioned processes-translation, transcription, replication and recombination-and discuss the significance of the results.