Table 2.

Predicted selectivity of PheRS in different cells and compartments

Cell type	PheRS	Cellular Phe∶Tyr*	Specificity Phe/Tyr†	Selectivity Phe/Tyr‡	Tyr-tRNAPhe editing	Viable in vivo
Yeast mitochondria
	mtPheRS	1.6∶1§	7,300	11,700	No	Yes
	A333G mtPheRS	1.6∶1	420	690	No	No
Yeast cytoplasm
	ctPheRS	4.6∶1¶	485	2,200	Yes	Yes
	D243A ctPheRS	4.6∶1	485	2,200	No	Yes
E. coli
	E. coli PheRS	1.9∶1∥	6,800	14,400	Yes	Yes
	A333G mtPheRS	1.9∶1	420	890	No	Yes

^*Ratio of concentrations of free Phe and Tyr.

^†See Table 1.

^‡Selectivity was defined as (specificity) × ([Phe]/[Tyr]), as applied to amino acid activation, but does not take into account possible posttransfer editing.

^§Estimate based on rat mitochondria (30).

^¶Total cellular amino acid pools (29).

^∥(31).