Ribosome profiling of porcine reproductive and respiratory syndrome virus reveals novel features of viral gene expression

The arterivirus porcine reproductive and respiratory syndrome virus (PRRSV) causes significant economic losses to the swine industry worldwide. Here we apply ribosome profiling (RiboSeq) and parallel RNA sequencing (RNASeq) to characterise the transcriptome and translatome of both species of PRRSV and to analyse the host response to infection. We calculated programmed ribosomal frameshift (PRF) efficiency at both sites on the viral genome. This revealed the nsp2 PRF site as the second known example where temporally regulated frameshifting occurs, with increasing -2 PRF efficiency likely facilitated by accumulation of the PRF-stimulatory viral protein, nsp1 beta. Surprisingly, we find that PRF efficiency at the canonical ORF1ab frameshift site also increases over time, in contradiction of the common assumption that RNA structure-directed frameshift sites operate at a fixed efficiency. This has potential implications for the numerous other viruses with canonical PRF sites. Furthermore, we discovered several highly translated additional viral ORFs, the translation of which may be facilitated by multiple novel viral transcripts. For example, we found a highly expressed 125-codon ORF overlapping nsp12, which is likely translated from novel subgenomic RNA transcripts that overlap the 3 & PRIME; end of ORF1b. Similar transcripts were discovered for both PRRSV-1 and PRRSV-2, suggesting a potential conserved mechanism for temporally regulating expression of the 3 & PRIME;-proximal region of ORF1b. We also identified a highly translated, short upstream ORF in the 5 & PRIME; UTR, the presence of which is highly conserved amongst PRRSV-2 isolates. These findings reveal hidden complexity in the gene expression programmes of these important nidoviruses.

Automated summary

This study characterizes the transcriptome and translatome of the arterivirus porcine reproductive and respiratory syndrome virus (PRRSV), and analyzes the host response to infection. The study reveals an increase in programmed ribosomal frameshift (PRF) efficiency over time at both viral genomic sites. Surprisingly, the study also finds an increase in PRF efficiency at the canonical ORF1ab frameshift site, contradicting the common assumption. Additionally, several highly translated additional viral ORFs and novel viral transcripts were discovered. These findings provide insights into the gene expression programs of PRRSV.