Assembly of Qβ viral RNA polymerase with host translational elongation factors EF-Tu and -Ts

Replication and transcription of viral RNA genomes rely on host-donated proteins. Q beta virus infects Escherichia coli and replicates and transcribes its own genomic RNA by Q beta replicase. Q beta replicase requires the virus-encoded RNA-dependent RNA polymerase (beta-subunit), and the host-donated translational elongation factors EF-Tu and -Ts, as active core subunits for its RNA polymerization activity. Here, we present the crystal structure of the core Q beta replicase, comprising the beta-subunit, EF-Tu and -Ts. The beta-subunit has a right-handed structure, and the EF-Tu: Ts binary complex maintains the structure of the catalytic core crevasse of the beta-subunit through hydrophobic interactions, between the finger and thumb domains of the beta-subunit and domain-2 of EF-Tu and the coiled-coil motif of EF-Ts, respectively. These hydrophobic interactions are required for the expression and assembly of the Q beta replicase complex. Thus, EF-Tu and -Ts have chaperone-like functions in the maintenance of the structure of the active Q beta replicase. Modeling of the template RNA and the growing RNA in the catalytic site of the Q beta replicase structure also suggests that structural changes of the RNAs and EF-Tu: Ts should accompany processive RNA polymerization and that EF-Tu: Ts in the Q beta replicase could function to modulate the RNA folding and structure.