The translation initiation site of satellite tobacco necrosis virus RNA does not require a start codon

Initiation of translation in eukaryotes is believed to occur by a scanning mechanism in which the 40s ribosomal subunit recognizes the 5$\sp\prime$ terminal cap structure (m$\sp7$G(5$\sp\prime$)ppp(5$\sp\prime$)N$\...$) and scans along the messenger RNA until an initiator AUG codon is found in a favorable context sequence ($\sp{\rm A}\sb{\rm G}$XXAUGG). Satellite Tobacco Necrosis Virus (STNV RNA) and the small component RNA of Alfalfa Mosaic Virus (AMV-4 RNA) are unique viral RNAs in that they are efficiently translated in the absence, or after removal of, 5$\sp\prime$-terminal m$\sp7$G(5$\sp\prime$)ppp(5$\sp\prime$)N groups. This suggests these mRNAs may employ alternate mechanisms to initiate translation than 5$\sp\prime$-terminal cap dependent mRNAs. In order to examine this theory, oligonucleotide directed site specific mutagenesis of pSTNV$\sb{1239}$ and pSP65-A4 plasmids containing full length cDNA clones of STNV RNA and AMV-4 RNA was used to generate single nucleotide substitutions in the translation initiation region of these mRNAs. Gel electrophoretic analyses of the $\sp{14}$C-proteins translated from STNV RNA, transcript STNV RNA, and mutant transcript STNV RNAs by an in vitro wheat germ system in the presence of 0.002 M Mg$\sp{++}$ show that a UUG, but not an AAG and AGG codon, can efficiently replace the translation initiator AUG of 5$\sp\prime$-terminally capped or capless transcript STNV RNAs. This phenomenon is not limited to normally 5$\sp\prime$-terminally uncapped mRNAs like STNV RNA for translation of transcript 5$\sp\prime$-terminally capped and uncapped AMV-4 RNAs containing a mutant UUG (Leu) codon in place of their translation initiator AUGs also initiates translation at, or near, the mutant UUG codon. The initiation of translation of these mRNAs at, or near, the site where the initiator AUG codon has been converted to UUG codon suggests that features other than the presence of a translation initiator AUG govern the initiation of translation of these viral mRNAs.U of I OnlyETDs are only available to UIUC Users without author permissio