Initiation of translation on nedicistrovirus and related intergenic region IRESs by their factor-independent binding to the P site of 80S ribosomes

Abstract

Initiation of translation on many viral mRNAs occurs by non-canonical mechanisms that involve 5’ end-independent binding of ribosomes to an internal ribosome entry site (IRES). The ~190 nt-long intergenic region (IGR) IRES of dicistroviruses such as Cricket paralysis virus (CrPV) initiates translation without Met-tRNAi^Met or initiation factors. Advances in metagenomics have revealed numerous dicistrovirus-like genomes with shorter, structurally distinct IGRs, such as Nedicistrovirus (NediV) and Antarctic picorna-like virus 1 (APLV1). Like canonical IGR IRESs, the ~165 nt-long NediV-like IGRs comprise three domains, but they lack key canonical motifs, including L1.1a/L1.1b loops (which bind to the L1 stalk of the ribosomal 60S subunit) and the apex of stemloop V (SLV) (which binds to the head of the 40S subunit). Domain 2 consists of a compact, highly conserved pseudoknot (PKIII) from which a CrPV-like stemloop SLIV and an adjacent conserved 5nt-long stemloop protrude. In vitro reconstitution experiments showed that NediV-like IRESs initiate translation from a non-AUG codon and form elongation-competent 80S ribosomal complexes in the absence of initiation factors and Met-tRNAi^Met. Unlike canonical IGR IRESs, NediV-like IRESs bind directly to the peptidyl (P) site of ribosomes leaving the aminoacyl (A) site accessible for decoding. The related structures of NediV-like IRESs and their common mechanism of action indicate that they exemplify a distinct class of IGR IRES.