FIG 10.

ProQ and Prc deficiencies alter ProP activity differently. E. coli strains RM2 (proQ⁺ prc⁺), WG1072 (RM2 ΔproQ756::kan [phenotypically ProQ⁻ Prc⁻]), WG1414 (WG1072 pDC77 [phenotypically ProQ⁺ Prc⁻]), WG703 (RM2 Δprc3::kan [phenotypically ProQ⁺ Prc⁻]), and WG1371 (WG703 pCK3 [phenotypically ProQ⁺ Prc⁺]) were cultivated in MOPS medium, and proline uptake rates were measured as a function of the assay medium osmolality, as described in Materials and Methods. Plasmid pDC77 encodes ProQ and pCK3 encodes Prc. For strains RM2 and WG703, nonlinear regression (performed with SigmaPlot) was used to fit the initial rate of proline uptake (a₀) at the corresponding assay medium osmolality (Π/RT) to the following equation: a₀ = A_max/[1 + e^−(Π-Π_½^)/RTB], where a₀ is the initial rate of substrate (radiolabeled proline) uptake, A_max is the rate that would be attained at infinite osmolality, B is a constant, Π_½/RT is the value of Π/RT at which a₀ = ½A_max, R is the gas constant (8.314 J · K⁻¹ · mol⁻¹), and T is the temperature (298°K). ProP activity was half-maximal at an osmolality of 284 ± 6 mmol/kg for RM2 and 323 ± 6 mmol/kg for WG703 (means ± standard errors).