Repellent taxis in response to nickel ion requires neither Ni2+ transport nor the periplasmic NikA binding protein

Abstract

Ni(2+) and Co(2+) are sensed as repellents by the Escherichia coli Tar chemoreceptor. The periplasmic Ni(2+) binding protein, NikA, has been suggested to sense Ni(2+). We show here that neither NikA nor the membrane-bound NikB and NikC proteins of the Ni(2+) transport system are required for repellent taxis in response to Ni(2+).

FIG. 1.

Distribution of wild-type and nik-knockout strains in aspartate and NiSO₄ gradients. Cells were exposed to gradients in a previously described microfluidic device (9). The dimensions of the observation chamber are 20 by 1,050 by 11,500 μm. (A) Response of CV1 to a 0 to 100 μM gradient of l-aspartate. Cells were introduced in the middle of the channel in the presence of 50 μM aspartate. The distribution of cells at a uniform concentration of 50 μM aspartate is shown for comparison. (B) Responses of strain CV1 to a 0 to 225 mM gradient of NiSO₄. In the gradient, cells were introduced in the middle of the channel in the presence of 122.5 μM NiSO₄. The distribution of cells at a uniform concentration of 122.5 μM NiSO₄ is shown for comparison. (C) Distribution of CV4 (Δtar) cells in gradients of 0 to 100 μM aspartate and 0 to 225 μM NiSO₄. Note that there is no significant attractant response to aspartate, but there is a residual repellent response to NiSO₄. (D) Responses of CV1 nikA and CV4 to a 0 to 225 μM gradient of NiSO₄. (E) Responses of CV1 nikB and CV4 to a 0 to 225 μM gradient of NiSO₄. (F) Responses of CV1 nikC and CV4 to a 0 to 225 μM gradient of NiSO₄. The same distribution for CV4 cells is shown in panels C to F. Cell counts for each were determined from 100 images taken over a 5-min interval from a point approximately 7 mm down the channel.

FIG. 2.

Images of cells responding to a NiSO₄ gradient in the microfluidic device. CV1, CV4, and CV1 nik-knockout strains were exposed to a gradient of 0 to 225 μM NiSO₄. Representative pseudocolored overlay images are shown. The high concentration is at the right side of the image. In all images, CV1 or CV1 nik-knockout cells are shown in green, CV4 cells are shown in red, and dead cells are shown in blue. (A) CV1; (B) CV1 nikA; (C) CV1 nikB; (D) CV1 nikC. Note that some of the CV1, CV1 nik, and CV4 cells always go in the “wrong” direction, toward higher concentrations of NiSO₄. Readers are reminded that chemotaxis occurs via a biased random walk, so that on a short time scale (18 to 21 s) some cells will randomly go toward higher NiSO₄ concentrations. The scale bar is 100 μm.

FIG. 3.

Distribution of CV12 (Tsr only) and CV13 (Tar only) strains in a NiSO₄ gradient. The assay was run as described in the legend to Fig. 1.