Continuous and discontinuous ribosome scanning on the cauliflower mosaic virus 35 S RNA leader is controlled by short open reading frames

The pathways of scanning ribosome migration controlled by the cauliflower m osaic virus 35 S RNA leader were investigated in vitro and in vivo. This lo ng (600 nucleotides) leader contains several short open reading frames (sOR Fs) and folds into an extended hairpin structure with three main stable ste m sections. Translation initiation downstream of the leader is cap-dependen t and occurs via ribosomal shunt under the control of two cis elements, a s hort open reading frame A (sORF A) followed by stem section 1. Here we show that a second similar configuration comprising sORF B followed by stem sec tion 2 also allows shunting. The efficiency of the secondary shunt was grea tly increased when stem section 1 was destabilized. In addition, we present evidence that a significant fi action of reinitiation-competent ribosomes that escape both shunt events migrate linearly via the structured central r egion but are intercepted by internal AUG start codons. Thus, expression do wnstream of the 35 S RNA leader is largely controlled by its multiple sORFs .