Table 2.

Autodephosphorylation Rate Constants for E. coli Response Regulator T+1 Mutants

Amino acid at T+1	kdephos (min−1)a
	CheBb	CheYc	NarLb
Ser	≥ 14d	5.2 ± 0.1	0.24 ± 0.02
Thr		5.0 ± 0.1
Ala	3.7 ± 0.04	3.2 ± 0.2	0.087 ± 0.01
Met		1.3 ± 0.1
Val	0.9 ± 0.1e	0.11 ± 0.01	0.030 ± 0.005
Ile		0.10 ± 0.01

^aMean and standard deviations of the observed first-order rate constants were determined from individual autodephosphorylation time courses. Rate constants for wild type proteins are underlined.

^bCheB and NarL dephosphorylation kinetics were measured directly by loss of ³²P. A second more variable assay inferred CheB autodephosphorylation as the rate-limiting step in the loss of ³²P from a 100-fold excess of CheA-P and gave rate constants that agreed within about two-fold (data not shown).

^cCheY dephosphorylation kinetics were measured by fluorescence using the pH jump method. Note the good agreement of these values with the y-intercepts of the best-fit lines in Figure 4 (plotted in units of s⁻¹), an alternative means of determining k_dephos.

^dAutodephosphorylation of wild type CheB is sufficiently rapid that measurement of k_dephos by loss of ³²P is technically challenging. This lower bound based on three time points before signal disappeared is consistent with a previous report using a different method.³⁵

^ePhosphotransfer from CheA-P to CheBS84V was slow (Figure 2B), which may result in a modest (≤ two-fold) underestimate for k_dephos.