The 5′ non-translated αβ-leader sequence of potato virus X RNA consists of two regions: the α sequence (41 nucleotides with no G) and the β sequence (42 nucleotides upstream from AUG). The αβ-leader has been shown to enhance strongly the expression of adjacent genes in chimeric mRNAs. This phenomenon has been postulated to be due to the unpaired conformation of the 5′-terminal 30 nucleotides and/or to the presence within the α region of the CCACC pentanucleotide complementary to the 3′-terminal conserved structure of 18S rRNA. Different derivatives of αβ-leader have been constructed for use in determining the contribution of separate elements of the αβ sequence to translational enhancement. It was found that deletion of the α sequence large fragment which was supposed to be unfolded did not reduce the Δαβ-leader enhancement activity. Moreover, translational enhancement was greater for this derivative. Deletion of the β sequence resulted in a considerable increase in activity of the α-leader showing that the β region was dispensable for translation. Disruption or ‘masking’ of CCACC led to inactivation of the αβ-leader as a translational enhancer. Thus, we identified the CCACC pentanucleotide as the primary motif responsible for the translation enhancing ability of αβ-leader.


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