We previously showed in intact yeast cells that the pseudoknot containing mRNA produced from pJDRC was able to promote efficient À1 PRF, whereas one in which only a stem-loop can form, transcribed from pJD18, could not (23) . As a first step in this study, we tested the ability of synthetic mRNAs produced from pJD366-RC and from pJD366-18, two plasmids derived from these parental constructs, to promote À1 PRF. Total luciferase activities produced from these synthetic mRNAs were divided by the luciferase activity produced from the zero-frame control plasmid, pJD366-0, and multiplied by 100% to determine À1 PRF efficiencies. The results show that the trends observed in yeast were replicated in vitro, i.e. JD366-RC mRNA promoted $8% efficiency of À1 PRF as compared with $1.1% promoted by JD366-18 mRNA (Figure 2 ).