Table II. Aspartylation of T.thermophilus tRNA^Asp and tRNA^Asn by native or engineered AspRSs.

AspRSs	tRNAAsp charging			tRNAAsn charging
	kcat s–1 (×10–2)	Km µM	kcat/Km (relative)	kcat s–1 (×10–2)	Km µM	kcat/Km (relative)
T.t.2-nativea	4.2	0.14	1	0.56	0.20	0.09
P.k.-native	1.17	0.2	0.17	no detectable charging
P.k.-mutantb	0.83	1.0	0.03	0.08	0.90	0.003

Aminoacylation conditions are as described in Materials and methods. K_m and k_cat values were determined at 37°C.

^aAt 70°C, the k_cat/K_m for tRNA^Asp is only 2-fold higher than for tRNA^Asn (Becker and Kern, 1998).

^bThe Pyrococcus mutant has the L1 loop exchanged with that from Thermus AspRS-2.