Left-handed to right-handed helix conversion in Salmonella flagella

Abstract

ALTHOUGH bacterial flagella can assume various helical configurations^1–3, the ‘normal’ and ‘curly’ configurations are encountered most frequently in nature. In Salmonella, normal and curly flagella have helical pitches of about 2.3 and 1.1 µm, respectively⁴. Macnab and Koshland⁵ observed that normal flagella of living Salmonella were left-handed helical filaments, as shown for Proteus and Bacillus⁶. Flagella with this handedness and rotating counterclockwise (looking in the direction of travel), would cause helical waves to propagate distally and provide forward thrust^7–9. Normal flagella can transform reversibly into the curly type when physiological conditions, such as pH, are varied^10,11: this has been termed biplicity. Asakura et al.¹², using Salmonella strain SJ670 and others, found that reconstituted flagellar filaments also could be transformed reversibly, although they could not control this transformation completely. SJ670 is a motile strain that produces normal flagella. On the other hand, the mutant SJ30 isolated by lino¹³ produces flagella which are stable in the curly configuration in various conditions. The mutation is in the structural gene for the flagellar protein subumit, flagellin, and the mutant cannot swim normally tout appears to tumble continuously. Now, using SJ670 and SJ30, we have found that, whereas the normal configuration of flagella is a left-handed helix, the curly configuration is a right-handed helix.