Table 1.

Aminoacylation of tRNAs by the E. coli, yeast (S. cerevisiae), and human cysteine-tRNA synthetases (CysRS)

CysRS	tRNAs (kcat/Km)
	eCys01	yCys01	hCys01	yCys13	hCys15
E. coli	1.00	0.01	0.01	0.28	1.71
Yeast	1.01	1.00	1.01	0.25	0.69
Human	0.61	0.24	1.00	0.19	0.31

Relative k_cat/K_m values compared to that of the homologous tRNA-synthetase interaction (underlined). The wild-type E. coli tRNA^Cys (eCys01), yeast tRNA^Cys (yCys01), and human tRNA^Cys (hCys01) serve as the homologous substrates for the E. coli, yeast, and human enzymes, respectively. The yeast tRNA mutant yCys13 and the human tRNA mutant hCys15 are two variants that are efficient substrates for the E. coli enzyme. All relative k_cat/K_m values are the average of at least three to five independent measurements. Data obtained for the E. coli enzyme are derived from measurements of k_cat and K_m values and have an average standard deviation of less than 10%. Data obtained for the yeast enzyme are derived from ratios of initial rates under conditions where tRNA substrates are 7- to 10-fold below Km values. These data have an average standard deviation of 30%. Data for the human enzyme are obtained from the HeLa cell lysate and are obtained by ratios of initial rates. These data are comparable to those obtained from the thioredoxin fusion (not shown) and have an average standard deviation of less than 10%.