Table 2.

PTS concentration and rate changes during chemotactic excitation

Response	[S] (nM)	t1/2 (s)	v (μM·s−1)	ΔEI	ΔPEP
Close to threshold	10	0.5	2.5	0.25	0.0025
Close to saturation	50	0.34	12	0.45	0.005
Monoexponential kinetics	>200	<0.1	>40

Fractional changes in PEP and phosphorelay components were computed from initial rates of transport, v = {[S]·V_max/([S]+ K_m)} μM·s⁻¹; where [S] = extracellularly photoreleased glucose; K_m was determined from Figure 5A. The fractional change, ΔC = (v·t_1/2)/C_T; where C_T, the total concentration, is 5, 25–100, and 25–50 μM for [EI], [HPr], and [EIIA], respectively (Scholte et al., 1982; Mattoo and Waygood, 1983), and 0.1–1 mM for [PEP] (Lowry et al., 1971). Therefore, ΔHPr and ΔEIIA^Glc are one-fifth to one-twentieth the ΔEI. This assumes that the components are initially present almost entirely in their phosphorylated forms and that back reactions are negligible during the excitation time period (see RESULTS). The latter assumption is likely to be valid for Δ[HPr] and Δ[EIIA], but will provide an underestimate for Δ[EI] or Δ[PEP]. V_max, hence v, were expressed as rates of micromolar change in intracellular concentration per second (μM·s⁻¹). Reported values expressed as μmol·g⁻¹ dry wt·min⁻¹ were converted using cytoplasmic volume/gram dry weight = (number of bacteria/gram dry weight) (volume/bacterium) = (6 × 10¹²) (1.4 × 10⁻¹²) ml/g = 8.4 ml/g·V_max for PTS d-glc transport = 200 μM·s⁻¹. In contrast, V_max for aspartate transport <1 μM·s⁻¹ (Kay, 1971). CheY.P dephosphorylation rate = (k_c + k_−c) [CheY.P] = (10 s⁻¹) (2 μM) = 20 μM·s⁻¹ (Jasuja et al., 1999). Thus, this will become the single rate-limiting process for PTS chemotactic signaling for 0.2 μM and greater concentration jumps.