Translational efficiency and competitive ability of mRNAs with 5'-untranslated alpha beta-leader of potato virus X RNA.

The 5'-untranslated leader sequence of potato virus X (PVX) RNA (63 nucleotides apart from cap-structure) consists of two sub-sequences referred to as alpha-sequence (41 nucleotides with no G) and beta-sequence (42 nucleotides upstream from the first AUG). Computer-based folding predictions suggest that the 5'-proximal region of alpha beta-leader is unstructured. The second structural feature of alpha beta-leader is the presence of the sequences apparently complementary to the 3'-terminal region of 18S rRNA. The alpha beta-leader has been shown to strongly enhance the translation of the contiguous foreign gene (NPT1) transcripts in cell-free translation systems from rabbit reticulocytes (RRL), wheat germ (WG) and Krebs-2 ascite cell extract (KA). In competitive translation PVX, RNA strongly inhibited tobacco mosaic virus (TMV) RNA, in RRL and WG systems. No competition occurred between PVX and TMV RNAs in KA system. There was no correlation between the translational efficiency and competitive ability of PVX RNA in different cell-free translation systems. The competitive ability did not solely depend on the presence of alpha beta-leader in mRNA. We present evidence to suggest that alpha beta-leader together with about 150 bases of the coding sequence is responsible for the translation competitive ability of PVX RNA.