Effects of changes in membrane sodium flux on virulence gene expression inVibrio cholerae

The expression of several virulence factors of Vibrio cholerae is coordinat ely regulated by the ToxT molecule and the membrane proteins TcpP/H and Tox R/S, which are required for toxT transcription, To identify proteins that n egatively affect toxT transcription, we screened transposon mutants of V ch olerae carrying a chromosomally integrated toxT:laci! reporter construct fo r darker blue colonies on media containing 5-bromo-4-chlor-3-indolyl beta-D gaLactoside (X-gal), Two mutants had transposon insertions in a region hom ologous to the nqr gene cluster of Vibrio alginolyticus, encoding a sodium- translocating NADH-ubiquinone oxidoreductase (NQR), In V alginolyticus, NQR is a respiration-linked Na+ extrusion pump generating a sodium motive forc e that can be used for solute import, ATP synthesis, and flagella rotation. Inhibition of NQR enzyme function in V cholerae by the specific inhibitor 2-n-heptyl-4-hydroxyquinoline N-oxide (HQNO) resulted in elevated toxT:laci ! activity. Increased toxT::lacZ expression in an nqr mutant strain compare d with the parental strain was observed when the TcpP/H molecules alone wer e strongly expressed, suggesting that the negative effect of the NQR comple x on toxT transcription is mediated through TcpP/H. However, the ability of the TcpP/H proteins to activate the toxT::lacZ reporter construct was grea tly diminished in the presence of high NaCl concentrations in the growth me dium. The flagellar motor of V: cholerae appears to be driven by a sodium m otive force, and modulation of flagella rotation by inhibitory drugs, high media viscosity, or specific mutations resulted in increases of toxT::lacZ expression, Thus, the regulation of the main virulence factors of I:, chole rae appears to be modulated by endogenous and exogenous sodium levels in a complex way.