The great variety of genome organizations means that most plant positive strand viral RNAs differ from the standard 5'-cap/3'-poly(A) structure of eukaryotic mRNAs. The cap and poly(A) tail recruit initiation factors that support the formation of a closed loop mRNA conformation, the state in which translation initiation is most efficient. We review the diverse array of cis-acting sequences present in viral mRNAs that compensate for the absence of a cap, poly(A) tail, or both. We also discuss the cis-acting sequences that control translation strategies that both amplify the coding potential of a genome and regulate the accumulations of viral gene products. Such strategies include leaky scanning initiation of translation of overlapping open reading frames, stop codon readthrough, and ribosomal frameshifting. Finally, future directions for research on the translation of plant positive strand viruses are discussed.

Positive strand RNA viral genomes are unique in the viral world in serving a dual role as mRNA and replicon. Since the origin of the minus-strand RNA replication intermediate is at the 3'-end of the genome, the 3'-untranslated region (UTR) clearly plays a role in viral RNA replication. The messenger role of this same RNA likely places functional demands on the 3'-UTR to serve roles typical of cellular mRNAs, including the regulation of RNA stability and translation. Current understanding indicates varied roles for positive strand RNA viral 3'-UTRs, with the dominant roles differing between viruses. Three case studies are discussed: turnip yellow mosaic virus RNA, whose 3' tRNA mimicry is thought to negatively regulate minus strand synthesis; brome mosaic virus, whose 3'-UTR contains a unique promoter element directing minus strand synthesis; and tobacco mosaic virus, whose 3'-UTR contains an enhancer of translational expression.

The genomes of many (+)-stranded RNA viruses, including plant viruses and alphaviruses, consist of polycistronic RNAs whose internal genes are expressed via subgenomic messenger RNAs. The mechanism(s) by which these subgenomic mRNAs arise are poorly understood. Based on indirect evidence, three models have been proposed: (1) internal initiation by the replicase on the (-)-strand of genomic RNA, (2) premature termination during (-)-strand synthesis, followed by independent replication of the subgenomic RNA and (3) processing by nuclease cleavage of genome-length RNA. Using an RNA-dependent RNA polymerase (replicase) preparation from barley leaves infected with brome mosaic virus (BMV) to synthesize the viral subgenomic RNA in vitro, we now provide evidence that subgenomic RNA arises by internal initiation on the (-)-strand of genomic RNA. We believe that this also represents the first in vitro demonstration of a replicase from a eukaryotic (+)-stranded RNA virus capable of initiating synthesis of (+)-sense RNA.

Transfer RNA-like structures (TLSs) that are sophisticated functional mimics of tRNAs are found at the 3′-termini of the genomes of a number of plant positive strand RNA viruses. Three natural aminoacylation identities are represented: valine, histidine, and tyrosine. Paralleling this variety in structure, the roles of TLSs vary widely between different viruses. For Turnip yellow mosaic virus, the TLS must be capable of valylation in order to support infectivity, major roles being the provision of translational enhancement and down-regulation of minus strand initiation. In contrast, valylation of the Peanut clump virus TLS is not essential. An intermediate situation seems to exist for Brome mosaic virus, whose RNAs 1 and 2, but not RNA3, need to be capable of tyrosylation to support infectivity. Other known roles for certain TLSs include (i) the recruitment of host CCA nucleotidyltransferase as a telomerase to maintain intact 3′ CCA termini, (ii) involvement in the encapsidation of viral RNAs, and (iii) presentation of minus strand promoter elements for replicase recognition. In the latter role, the promoter elements reside within the TLS but are not functionally dependent on tRNA mimicry. The phylogenetic distribution of TLSs indicates that their evolutionary history includes frequent horizontal exchange, as has been observed for protein-coding regions of plant positive strand RNA viruses.

Dengue fever-dengue hemorrhagic fever (DF-DHF) is a complex of clinical syndromes caused by dengue (DEN) viruses of the genus Flavivirus (family Flaviviridae). DF-DHF has become a major global health problem over the past 25 years, producing more human illness than any other insect-transmitted viral disease (7). The geographic distribution of DEN viruses, their mosquito vectors (primarily Aedes aegypti), and the resulting disease burden continues to increase. The World Health Organization estimates that over 50 million new human infections occur annually. DF-DHF has become a leading cause of hospitalization and death among children in Southeast Asia (31), and the incidences of infection and disease are sharply rising in the Americas (8). There is currently no commercially available vaccine, chemoprophylactic, or effective therapeutic. Medical supportive care is the recommended primary treatment modality to improve the condition of severely infected patients (20).The DEN virus genome is an approximately 10.7-kb singlestranded RNA of positive polarity with a 5Ј cap and lacking a 3Ј poly(A) tail. DEN virus genomic RNA has a single long open reading frame (ORF) encoding three structural and seven nonstructural proteins. The mature DEN virus proteins are derived by co-or posttranslational cleavage of the polyprotein encoded by the ORF. The ORF is flanked by 5Ј and 3Ј untranslated regions (5Ј and 3Ј UTRs) that have various functions in the initiation and regulation of viral translation, replication, and assembly (21,24,34).Serologic and molecular analyses have defined four antigenically distinct serotypes of DEN virus (DEN virus serotype 1 [DEN-1 virus], DEN-2 virus, DEN-3 virus, and DEN-4 virus), which exhibit overall amino acid sequence identity of approximately 70% (12). Each serotype, in turn, contains several genotypic strains. The four DEN virus serotypes have pantropical distributions and cause similar spectra of clinical conditions (7). Persons who have experienced a single DEN virus infection generally develop a long-lasting immunity to DEN virus strains of the same serotype. However, subsequent infection with a different DEN virus serotype has a likelihood of leading to severe disease, at least in part through an immunologic process known as antibody-dependent enhancement (10,18,27). It is essential that a prospective vaccine and highly desirable that a prospective therapeutic be effective against all four DEN virus serotypes. Whether or not a safe and efficacious tetravalent DEN virus vaccine becomes available in the near future, there remains a critical need to develop effective therapeutics to treat patients suffering from severe DEN virus infection.Antisense compounds of various structural types have been successfully used to affect gene expression in several viral pathogens of humans (23). An antisense therapeutic for cytomegalovirus retinitis that targets the cytomegalovirus immediate-early gene 2 product has received approval from the Food and Drug Administration (25), and a number of oligonucleotide ...