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 mum (BvgS) and 4 mum (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 qui- none-dependent inhibition of autophosphorylation. These data suggest that BvgS and EvgS are connected with the oxi- dation status of the cell via the link to the ubiquinone pool.