Normal-to-curly flagellar transitions and their role in bacterial tumbling. Stabilization of an alternative quaternary structure by mechanical force.

Tumbling, a characteristic feature of the motility and chemotactic response of peritrichously flagellated bacteria, has been examined by dark-field light microscopy and found to be intimately linked to major changes in the quaternary structure of the flagella. The normal structure of the flagella of Salmonella is a left-handed helix of wavelength 2·3 μm and diameter 0·4 μm. Under right-handed torsion generated when the rotary motor is operating in reverse (i.e. in a clockwise sense), the normal structure undergoes a discrete transition to an alternative structure, designated “curly”, which is a right-handed helix of wavelength 1·1 μm and diameter 0·3 μm. The transition proceeds rapidly from the cell body outward but is, at any instant, confined to a very short region of the flagellum. The heteromorphous structure of a flagellum in transition consists of two portions, normal and curly, with a change in axial direction of 64° occurring at the transition region. The chaotic motion of the cell body, in reaction to a number of flagella which are rotating and in transition, constitutes the tumble. The induction of polymorphic transitions by mechanical force is confirmed by studies with motor and flagellar mutants, and with cells converted to curly flagellar phenotype by p -fluorophenylalanine. The phenomenon of polymorphic transitions was examined most extensively in Salmonella , but is also noted in Bacillus subtilis and Escherichia coli and may therefore be a general feature of bacterial flagellar structure. The right-handedness of the curly structures induced mechanically or by incorporation of p -fluorophenylalanine is also found for the curly structures caused by mutation or pH/ionic strength changes (Shimada et al. , 1975). The reasons for the specific interaction between mechanical and molecular forces are discussed.