Fig 1.
Assay system. (A) Schematic showing mutagenesis of the ATG start codon of the ung gene using the plasmid pTrc-ung as the template. The list shows the various combinations generated by random mutagenesis of the start ATG site. The first (AUG) and the fourth (GAA) codons in the ung gene are shown. The ung ORF was cloned into pTrc99c to generate pTrc-ung, and hence the mRNA bears the Shine-Dalgarno (S.D.) sequence of the expression vector. (B) Schematic representation of the elongator tRNAs used in this study and the changed start codons corresponding to them. The third schematic in the top row shows a mismatched pairing of tRNAGly (CCC anticodon) with the GGU codon for Gly. The anticodon and the region corresponding to the 3 G-C base pairs in the initiator tRNA anticodon stem are indicated.