NONCANONICAL TRANSLATION MECHANISMS IN PLANTS - EFFICIENT IN-VITRO AND IN PLANTA INITIATION AT AUU CODONS OF THE TOBACCO MOSAIC-VIRUS ENHANCER SEQUENCE

The 5' untranslated leader (Omega sequence) of tobacco mosaic virus (T MV) genomic RNA was utilized as a translational enhancer sequence in e xpression of the 17 kDa putative movement protein (pr17) of potato lea f roll luteovirus (PLRV). In vitro translation of RNAs transcribed fro m appropriate chimeric constructs, as well as their expression in tran sgenic potato plants, resulted in the expected wild-type pr17 protein, as well as in larger translational products recognized by pr17-specif ic antisera, Mutational analyses revealed that the extra proteins were translated by non-canonical initiation at AUU codons present in the w ild-type Omega sequence, In the plant system translation initiated pre dominantly at the AUU codon at positions 63-65 of the Omega sequence, Additional AUU codons in a different reading frame of the Omega sequen ce also showed the capacity for efficient translation initiation in vi tro. These results extend the previously noted activity of the TMV 5' leader sequence in ribosome binding and translation enhancement in tha t the TMV translation enhancer can mediate non-canonical translation i nitiation in vitro and in vivo.