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2002
Book Article
Titel
Gene expression strategies of RNA viruses
Abstract
Plant viral genomes encode a limited set of proteins that are required for viral replication, particle formation, movement of the virus or its RNA from cell to cell, polyprotein maturation, and interaction of the virus with its vector or host. Because of their limited genomic size, plant RNA viruses exploit strategies for gene expression that are not commonly used in the plant kingdom. This includes transcriptional strategies such as the production of subgenomic RNAs (sgRNAs). SgRNAs are messenger RNAs (mRNAs) coding for the 3'-proximal genes of a polycistronic viral RNA and therefore are identical copies of the corresponding region on the viral genome (genomic RNA [gRNA]). When several genes are present at the 3'-end of the gRNA, a family of 3'-colinear sgRNAs are produced such that each expressed gene is located at the 5'-end of one sgRNA, a situation that is required for proper translation in eukaryotes. At the level of translation initiation, leaky scanning of the initiation complex allows expression of more than one protein from the same RNA. In this mechanism, a portion of the 40S ribosomal subunit bypasses the 5'-proximal AUG and initiates at a downstream AUG (Kozak, 1989c), if this is placed in a better context for initiation of translation. One alternative to this distal entry of part of the translational machinery and its linear migration is the internal entry of ribosomes, guided by "internal ribosome entry sites" (IRES; Herman, 1989). During protein elongation, ribosomal frameshifting allows the production of two proteins (the frame protein and the transframe protein) that are identical at the N-terminus but differ downstream from the frameshifting point. The frame proteins are more abundant than transframe proteins since ribosomal frameshifting occurs at only very low frequencies. At the termination step, suppression of a stop codon involving specific suppressor transfer RNAs (tRNAs) produces a carboxy-terminal (C-terminal) extended protein. In addition, viruses use expression of a polyprotein precursor that is cleaved into different proteins using the protease activity of one or more of the viral products (Matthews, 1991; Maia et al., 1996). For most of these expression strategies the underlying mechanism is well understood and reviewed elsewhere (Rohde et al., 1994; Maia et al., 1996; Drugeon et al., 1999). In this chapter, we focus on novel aspects of plant virus gene expression, and, therefore, strategies such as translation initiation mediated by "internal ribosomal entry sites" are highlighted.