EFFECTS OF SEQUENCE ELEMENTS IN THE POTATO VIRUS-X RNA 5' NON-TRANSLATED ALPHA-BETA-LEADER ON ITS TRANSLATION ENHANCING ACTIVITY

The 5' non-translated alphabeta-leader sequence of potato virus X RNA consists of two regions: the alpha sequence (41 nucleotides with no G) and the beta sequence (42 nucleotides upstream from AUG). The alphabe ta-leader has been shown to enhance strongly the expression of adjacen t genes in chimeric mRNAs. This phenomenon has been postulated to be d ue to the unpaired conformation of the 5'-terminal 30 nucleotides and/ or to the presence within the alpha region of the CCACC pentanucleotid e complementary to the 3'-terminal conserved structure of 18S rRNA. Di fferent derivatives of alphabeta-leader have been constructed for use in determining the contribution of separate elements of the alphabeta sequence to translational enhancement. It was found that deletion of t he alpha sequence large fragment which was supposed to be unfolded did not reduce the DELTAalphabeta-leader enhancement activity. Moreover. translational enhancement was greater for this derivative. Deletion of the beta sequence resulted in a considerable increase in activity of the alpha-leader showing that the beta region was dispensable for tran slation. Disruption or 'masking' of CCACC led to inactivation of the a lphabeta-leader as a translational enhancer. Thus, we identified the C CACC pentanucleotide as the primary motif responsible for the translat ion enhancing ability of alphabeta-leader.