In the absence of a 5鈥?cap, plant positive-strand RNA viruses have evolved a number of different elements in their 3鈥?untranslated region (UTR) to attract initiation factors and/or ribosomes to their templates. These 3鈥?cap-independent translational enhancers (3鈥?CITEs) take different forms, such as I-shaped, Y-shaped, T-shaped, or pseudoknotted structures, or radiate multiple helices from a central hub. Common features of most 3鈥?CITEs include the ability to bind a component of the translation initiation factor eIF4F complex and to engage in an RNA-RNA kissing-loop interaction with a hairpin loop located at the 5鈥?end of the RNA. The two T-shaped structures can bind to ribosomes and ribosomal subunits, with one structure also able to engage in a simultaneous long-distance RNA-RNA interaction. Several of these 3鈥?CITEs are interchangeable and there is evidence that natural recombination allows exchange of modular CITE units, which may overcome genetic resistance or extend the virus's host range.
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