Application of Zinpyr-1 for the investigation of zinc signals in Escherichia coli

Abstract

Changes of the pico- to nanomolar concentration of free intracellular Zn²⁺ are part of the signal transduction in mammalian cells. These zinc signals regulate the enzymatic activity of target proteins such as protein tyrosine phosphatases. For Escherichia coli, previous studies have reported diverging concentrations from femto- to picomolar, raising the question if Zn²⁺ could also have a function in bacterial signaling. This manuscript explores the use of the low molecular weight fluorescent probe Zinpyr-1 in E. coli. The probe detects free Zn²⁺ in these bacteria. Comparable to mammalian cells, other metal ions, especially Hg²⁺ and Cd²⁺, interfere with the detection of Zn²⁺. Moreover, experiments in E. coli were particularly prone to artifacts based on cellular autofluorescence, necessitating corrections that are not required in mammalian cells. Based on measurements in lysates of E. coli and the mammalian cell line Jurkat, similar values between 0.1 and 0.2 nM free Zn²⁺ were found. For E. coli, this corresponds to less than one free zinc ion per cell. Moreover, phosphatase inhibition by Zn²⁺ was only observed in Jurkat, but not E. coli. This excludes a function for zinc signals as a regulator of bacterial phosphatases. Still, changes in the free Zn²⁺ concentration were observed in response to elevated extracellular Zn²⁺ and pH, or to addition of the detergent NP-40, suggesting that other processes could be controlled by the free intracellular Zn²⁺ concentration.