Ribosome Shunting in Cauliflower Mosaic Virus
Translation of Cauliflower mosaic virus (CaMV) 35S RNA is initiated by a ribosome shunt. The 35S RNA of CaMV contains a ~600nt leader sequence which contains 7-9 short open reading frames (sORFs) depending on the strain. This long leader sequence has the potential to form an extensive complex stem-loop structure, which is an inhibitory element for expression of following ORFs. However, translation of ORFs downstream of the CaMV 35S RNA leader has been commonly observed. Ribosome shunting model indicates with the collaboration of initiation factors, ribosomes start scanning from capped 5’-end and scans for a short distance until it hits the first sORF. The hairpin structure formed by leader brings the first long ORF into the close spatial vicinity of a 5’-proximal sORF. After read through sORF A, the 80S scanning ribosome disassembles at the stop codon, which is the shunt take-off site. The 40S ribosomal subunits keep combining with RNA, and bypass the strong stem-loop structural element, land at the shunt acceptor site, resume scanning and reinitiate at the first long ORF. 5’-proximal sORF A and the stem-loop structure itself are two essential elements for CaMV shunting . sORFs with 2-15 codons, and 5-10 nts between sORF stop codon and the base of the stem structure are optimal for ribosome shunting, while the minimal (start-stop) ORF does not promoter shunting.
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