Gene sequence, overproduction, purification and determination of the wild-type level of the Escherichia coli flagellar switch protein FliG.

The flagellar motor switch in Escherichia coli and Salmonella typhimurium controls swimming behavior by regulating the direction of flagellar rotation. The switch is a complex apparatus composed of at least three proteins--FliG, FliM and FliN. During chemotactic behavior, the switch responds to signals transduced by the chemotaxis sensory signaling system. CheY, the chemotaxis response regulator, is thought to act directly on the switch to induce tumbles in the swimming pattern, but physical interaction of CheY and switch proteins has not been shown. We have undertaken this work to develop the molecular tools to investigate CheY binding to switch proteins, as well as to understand more about the structure and function of the switch. We present here the sequences of the fliG gene and its protein product, the engineering and amplification of fliG by the polymerase chain reaction (PCR) and its subcloning, and the overproduction, purification and determination of the wild-type (wt) level of the FliG protein. The sequence data revealed a 91.8% amino acid (aa) identity between E. coli and S. typhimurium FliG. Engineering and amplifying fliG by PCR allowed convenient cloning into an efficient expression vector. FliG was successfully overproduced and purified to > 98% purity. Polyclonal antibodies (Ab) were generated against purified FliG and used in quantitative Western blots to determine that the wt expression level of fliG results in about 3700 FliG copies per cell. Purified FliG and anti-FliG Ab will be useful for direct biochemical analyses of CheY-switch protein interaction.