Two Alternative Ways of Start Site Selection in Human Norovirus Reinitiation of Translation [Protein Synthesis and Degradation]

March 5th, 2014 by Luttermann, C., Meyers, G.

The calicivirus minor capsid protein VP2 is expressed via termination/reinitiation. This process depends on an upstream sequence element denoted termination upstream ribosomal binding site (TURBS). We have shown for feline calicivirus (FCV) and rabbit hemorrhagic disease virus (RHDV) that the TURBS contains three sequence motifs essential for reinitiation. Motif1 is conserved among caliciviruses and complementary to a sequence in the 18S rRNA leading to the model that hybridization between motif1 and 18S rRNA tethers the post-termination ribosome to the mRNA. Motif2 and motif2* are proposed to establish a secondary structure positioning the ribosome relative to the start site of the 3′-terminal ORF. Here we analyzed human norovirus (huNV) sequences for the presence and importance of these motifs. The 3 motifs were identified by sequence analyses in the region upstream of the VP2 start site and we showed that these motifs are essential for reinitiation of huNV VP2 translation. More detailed analyses revealed that the site of reinitiation is not fixed to a single codon and does not need to be an AUG, even though this codon is clearly preferred. Interestingly, we were able to show that reinitiation can occur at AUG codons downstream of the canonical start/stop site in huNV and FCV, but not in RHDV. While reinitiation at the original start site is independent of the Kozak context, downstream initiation exhibits requirements for start site sequence context known for linear scanning. These analyses on start codon recognition give a more detailed insight into this fascinating mechanism of gene expression.