Insect viruses have evolved strategies to control the host RNAi antiviral defense mechanism. In nature Drosophila C Virus (DCV) infection causes low mortality and persistent infection, whereas the closely related Cricket Paralysis Virus (CrPV) causes a lethal infection. We show these viruses use different strategies to modulate the host RNAi defense machinery. The DCV RNAi suppressor (DCV-1A) binds to long double-stranded RNA (dsRNA) and prevents processing by Dicer2. In contrast, the CrPV suppressor (CrPV-1A) interacts with the endonuclease Ago2 and inhibits its activity, without affecting the miRNA-Ago1 mediated silencing. The link between viral RNAi suppressors and the outcome of infection was examined using recombinant Sindbis viruses encoding either CrPV-1A or DCV-1A. Flies infected with Sindbis virus expressing CrPV-1A showed a dramatic increase in virus production, spread and mortality. In contrast, Sindbis pathogenesis was only modestly increased by expression of DCV- 1A. We conclude that RNAi suppressors function as virulence factors.

Two classes of viruses, namely members of the Potyviridae and Caliciviridae, use a novel mechanism for the initiation of protein synthesis that involves the interaction of translation initiation factors with a viral protein covalently linked to the viral RNA, known as VPg. The calicivirus VPg proteins can interact directly with the initiation factors eIF4E and eIF3. Translation initiation on feline calicivirus (FCV) RNA requires eIF4E because it is inhibited by recombinant 4E-BP1. However, to date, there have been no functional studies carried out with respect to norovirus translation initiation, because of a lack of a suitable source of VPg-linked viral RNA. We have now used the recently identified murine norovirus (MNV) as a model system for norovirus translation and have extended our previous studies with FCV RNA to examine the role of the other eIF4F components in translation initiation. We now demonstrate that, as with FCV, MNV VPg interacts directly with eIF4E, although, unlike FCV RNA, translation of MNV RNA is not sensitive to 4E-BP1, eIF4E depletion, or foot-and-mouth disease virus Lb protease-mediated cleavage of eIF4G. We also demonstrate that both FCV and MNV RNA translation require the RNA helicase component of the eIF4F complex, namely eIF4A, because translation was sensitive (albeit to different degrees) to a dominant negative form and to a small molecule inhibitor of eIF4A (hippuristanol). These results suggest that calicivirus RNAs differ with respect to their requirements for the components of the eIF4F translation initiation complex.

The 5 terminus of picornavirus genomic RNA is covalently linked to the virus-encoded peptide 3B (VPg). Foot-and-mouth disease virus (FMDV) is unique in encoding and using 3 distinct forms of this peptide. These peptides each act as primers for RNA synthesis by the virus-encoded RNA polymerase 3D pol . To act as the primer for positive-strand RNA synthesis, the 3B peptides have to be uridylylated to form VPgpU(pU). For certain picornaviruses, it has been shown that this reaction is achieved by the 3D pol in the presence of the 3CD precursor plus an internal RNA sequence termed a cis-acting replication element (cre). The FMDV cre has been identified previously to be within the 5 untranslated region, whereas all other picornavirus cre structures are within the viral coding region. The requirements for the in vitro uridylylation of each of the FMDV 3B peptides has now been determined, and the role of the FMDV cre (also known as the 3B-uridylylation site, or bus) in this reaction has been analyzed. The poly(A) tail does not act as a significant template for FMDV 3B uridylylation.Picornaviruses, including foot-and-mouth disease virus (FMDV), poliovirus (PV), and human rhinoviruses (HRVs), have a positive-sense RNA genome of about 8 kb that is infectious (1, 5). Following virus attachment and entry into the cell, the RNA genome is delivered into the cytoplasm and translation of the genome is required to produce the viral proteins that are necessary for virus assembly and RNA replication. The genome encodes a single large polyprotein that is processed, largely by internal trans-acting proteases, to produce about 12 mature proteins plus various precursors (some of these have distinct functions). The proteins encoded within the P1 region form the capsid, while proteins encoded in the P2 and P3 regions are required for RNA replication. After some rounds of translation, there has to be a switch so that translation of the viral RNA is stopped and RNA replication can commence, since these two processes appear incompatible on the same RNA molecule (11). Most picornaviruses replicate with high efficiency within susceptible cells, and within a few hours, the amount of viral RNA can represent 5% of the total RNA in cells (a level similar to that of all the cellular cytoplasmic mRNAs together). Nearly all of the FMDV RNA generated by replication is infectious; indeed, microinjection of cells with as little as 1 to 2 molecules of viral RNA is sufficient to initiate an infection (4). Thus, the replication of FMDV genomes within cells is remarkably efficient.To replicate the positive-sense genome, an antisense RNA has to be synthesized which then functions as the template for the production of new positive-sense infectious genomes (32). RNA is synthesized by the viral 3D protein that functions as an RNA-dependent RNA polymerase and will be referred to as 3D pol . Interestingly, 3D pol requires the uridylylated form of the 3B/VPg peptide (VPgpU or VPgpUpU) to act as the primer for both positive-and negative-strand synthesis. In recent...