CP1 addition to hybrid M. mobile LeuRS enhances pretransfer editing. Reaction mixture contained 1 μM enzyme, 10 μM tRNALeu, or tRNALeu with A76 replaced by dideoxyadenosine (ddA), 18.1 μM [α-32P]ATP (40 μCi/mL), and 2.5 mM isoleucine. The AMP formation activities are measured for Mm LeuRS (A), MmLeuRS/CP1Leu (Mm/ CP1Leu) (B), MmLeuRS/CP1Ile (Mm/CP1Ile) (C), and MmLeuRS/CP1Val (Mm/CP1Val) (D). Symbols used are as follows: ◆, reaction without amino acid present (no AA); ▲, isoleucine (Ile); ▼, isoleucine with tRNA (Ile+tRNA); and ■, isoleucine with ddtRNA (Ile+ddA). Error bars represent the SD values based on three separate experiments.