Thermal transition in helical forms of Salmonella flagella

Flagellar filaments from three strains of Salmonella undergo reversible structural transitions between discrete helical forms when the temperature is changed under appropriate aqueous conditions. These transitions can be quantified by viscometry of concentrated flagellar solutions. A transition resulting in an increase in the right-handed structural twist of the filament is always exothermic. The change in van't Hoff enthalpy accompanying the transformation ranged between 90 kcal and 250 kcal per co-operative unit depending on the type of transformation. From the relation between the transition temperature and the pH, it is inferred that binding of five to six protons to a co-operative unit is involved in the transformations of two kinds of flagella.When small amounts of flagellins from straight mutants are incorporated into normal flagellar filaments by copolymerization, the transition temperature either increases or decreases, depending on the species of the mutant flagellins, as compared with that in the normal homopolymers. These results are discussed in terms of two-state models of flagellar polymorphism (Calladine, 1978; Kamiya et al., 1979).