Figure 4.

Adenylate turnover by wild type HisRS in the presence and absence of non-cognate tRNA^Trp at 37 °C and pH 7.5. [α-³²P]-AMP formation was measured in the presence (open circles and broken line) and absence (solid circles and solid line) of non-cognate tRNA^Trp. The experiment was performed as described in “Experimental Procedures.” The progress curves were fit to a bi-exponential followed by a linear phase. The exponentials represent the rates of formation of adenylate in active sites one and two, followed by a slow linear rate that is likely represents reiterative cycles of adenylate formation, hydrolysis, and product dissociation in site two.