Role of Fumarate in the Operation of the Bacterial Flagellar Motor

Abstract

Fumarate has previously been identified as a ‘switch factor’ that can alter the switching frequency and bias of the sense of rotation of the bacterial flagellar motor; however, despite numerous studies during the past fifty years, the molecular mechanism by which fumarate causes these effects could not be understood. We studied the motility of Salmonella by bright field and by monitoring rotation in a tethered cell assay. We carried out a detailed computerized analysis of the swimming and rotational behavior in real time under various conditions of external pHs (5.0 ± 8.0) and added fumarate concentrations (0 ± 12 mM). Based on our experimental results, we make the novel proposal that the bacterial flagellar motor contains access channels permeable to the fumarate monoanion, and that it catalyzes the overall electroneutral transport of fumarate and protons. We further postulate that each ionic species provides ∼50% of the total energy for rotation of the bacterial flagellar motor. These predictions are applicable to both the normal (say CW) mode of motor rotation as well as to the so-called “switched” (CCW) sense of rotation, depending on the direction of translocation of the ions. Hence previous proposals of fumarate as a ‘switch factor’, are false, or at best incomplete, because fumarate participates in the rotation of the flagellar motor in both normal and switched modes of operation. The new hypothesis extends to bacterial motility the essential and vital role played by dicarboxylic acid anions in oxidative phosphorylation and photosynthesis according to Nath's torsional mechanism of energy transduction and ATP synthesis (Nath, S., Beyond the Chemiosmotic Theory, Journal of Bioenergetics and Biomembranes 2010,42, 301-309).