The unorthodox histidine kinases BvgS and EvgS are responsive to the oxidation status of a quinone electron carrier.

The purified soluble forms of the histidine kinases BvgS and EvgS of Bordetella pertussis and Escherichia coli, respectively, are shown to be responsive to oxidized ubiquinone-0 (Q-0) in vitro. The oxidized ubiquinone is a strong inhibitor of kinase activity of both enzymes with half maximal inhibition occurring at 11 microm (BvgS) and 4 microm (EvgS). Reduced Q-0 has no effect on the histidine kinases. Kinase activity can reversibly be switched off and on by changing the oxidation status of the quinone. This inhibitory effect is due to a decrease of the kinase activity of BvgS rather than an increase of intrinsic phosphatase activities. Other electron carriers such as menadione (MK-3), NAD or FAD did not have a significant effect on the kinase activities of BvgS and EvgS. Nicotinic acid and sulfate ions, known to inhibit the histidine kinases in vivo, did not affect the purified truncated sensor proteins lacking their periplasmic domains in vitro. Mutations introduced by site-directed mutagenesis into the putative PAS domain of BvgS caused a weak decrease of quinone-dependent inhibition of autophosphorylation. These data suggest that BvgS and EvgS are connected with the oxidation status of the cell via the link to the ubiquinone pool.