Formation of helical filaments by copolymerization of two types of 'straight' flagellins.

Bacterial flagella undergo transition between several discrete types of left-handed and right-handed helical structures when exposed to acidic or alkaline pH, or to mechanical force. Calladine and ourselves have presented models to explain such polymorphism, assuming that protein subunits (flagellin) in a flagellum can be transformed into two conformations (L- and R-states) depending on the species of flagellin and on the environmental conditions. An obvious prediction from these 'two-state' models is that there should be two types of straight flagella (L- and R-types) that are made up exclusively of flagellins in either the L-state or the R-state. We have shown that straight flagella from two species of mutants, Salmonella SJ814 (ref. 6) and Escherichia coli hag 177 (ref. 7), are closely similar to the predicted R- and L-types, respectively. Recently we have isolated 10 strains of straight-flagellar mutants of Salmonella. We show here that their flagella can also be classified into the L- and R-types, and that copolymerization of flagellins from two heterologous types (L and R) makes discrete types of helical filaments, whereas that of homologous pairs of flagellins (L and L, or R and R) makes only straight filaments.