Control of tumbling in bacterial chemotaxis by divalent cation

Chemotaxis is migration of organisms to higher concentrations of attractant or lower concentrations of repellent. Understanding the switch that controls whether the flagella rotate counterclockwise for swimming or clockwise for tumbling (thrashing about without making much forward progress) is central to understanding chemotaxis of peritrichous bacteria, since chemotaxis results from selective suppression of tumbles. Depletion of divalent cation by chelating agents in the presence of A23187, an ionophore that conveys divalent cation across membrane, causes incessant tumbling in Bacillus subtilis. Small additions of MgCl₂ prevent this tumbling. In this tumbling condition, the bacteria, which normally swim extensively when given attractant, do not respond even to 10 mM alanine, a strong attractant. MnCl₂, by contrast to other divalent cations, increases tumbling in the absence of A23187, and the effect is potentiated by the ionophore. Permeant cations, including tetraphenylarsonium ion and triphenylmethylphosphonium ion, cause permanent swimming, even in the presence of A23187 and chelating agents. We propose that divalent cation, probably Mg²⁺ ion, binds to the switch to cause swimming and that, in the absence of divalent cation at the switch, the bacterium tumbles.