LUTEOVIRUS GENE-EXPRESSION

The genome of a luteovirus consists of a positive-sense RNA that is 5.5 to 5.7 kb long and encodes up to six open reading frames. The functions of few of these genes have been identified with certainty. Possible functions of all luteoviral genes and their roles in the viral life cycle are discussed. A variety of unusual translation events, as well as RNA-templated transcription, control expression of luteoviral genes. The translational control mechanisms include ribosomal frameshifting, stop codon suppression, leaky scanning initiation, and, in one case, cap-independent translation. The cis acting sequences and secondary structures that facilitate these events have been identified to varying degrees. Viral genes are expressed from one or two of the as many as three subgenomic RNAs that have been identified. Luteoviruses fall into two subgroups that have different genome organizations and may employ quite different replication mechanisms. A model for the origin of luteoviral subgroups by replicase strand-switching at subgenomic RNA promoters is presented.