3′ Cap-Independent Translation Enhancers of Plant Viruses

In the absence of a 5 ' cap, plant positive-strand RNA viruses have evolved a number of different elements in their 3 ' untranslated region (UTR) to attract initiation factors and/or ribosomes to their templates. These 3 ' cap-independent translational enhancers (3 ' 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 ' 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 ' 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 ' 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.