RIBOSOME SHUNTING IN CAULIFLOWER MOSAIC-VIRUS - IDENTIFICATION OF AN ESSENTIAL, AND SUFFICIENT STRUCTURAL ELEMENT

A wheat germ cell-free system was used to study details of ribosome sh unting promoted by the cauliflower mosaic virus 35 S RNA leader. By te sting a dicistronic construct with the leader placed between two codin g regions, we confirmed that the 35 S RNA leader does not include an i nternal ribosome entry site of the type observed with picornavirus RNA s. A reporter gene fused to the leader was shown to be expressed by ri bosomes that had followed the bypass route (shunted) and, with lower e fficiency, by ribosomes that had scanned through the whole region. Ste m section 1, the most stable of the three stem sections of the leader, was shown to be an important structural element for shunting. Mutatio ns that abolished formation of this stem section drastically reduced r eporter gene expression, whereas complementary mutations that restored stem section 1 also restored shunting. A micro-leader capable of shun ting consisting of stem section 1 and flanking sequences could be defi ned. A small open reading frame preceding stem section 1 enhances shun ting.