Point mutations that lock Salmonella typhimurium flagellar filaments in the straight right-handed and left-handed forms and their relation to filament superhelicity

We have determined the nucleotide sequence of two mutant and the parent fliC genes, encoding the protein flagellin (serotype i), of Salmonella typhimurium. The flagellar filaments of the two mutants, SJW1655 and SJW1660, are locked in the straight-right-handed ( R) and straight-left-handed ( L) conformations, respectively. Their normal, wild-type, parent strain is SJW1103. These mutant strains differ from the wild-type by only one base-pair: the mutation of SJW1655 occurs at nucleotide 1346 in the flagellin gene, changing a C · G pair to T · A (alanine 449 to valine). The mutation of SJW1660 occurs at nucleotide 1277, changing a G · C pair to C · G (glycine 426 to alanine). The resulting amino acid substitutions are near the C terminus predicted to form an α-helical coiled coil. The region contains six heptad repeats. Similar α-helical segments (three and four repeats long) are present near the N terminus. Alignment of the 17 flagellin sequences available to date confirms the generality of these segments. The mutations are within that portion of the sequence assigned, by proteolytic cleavage, to the middle flagellin domain whose length corresponds to the six heptad repeats found in the sequence (~50 Å). We have shown that these mutations are the sole cause of the straight phenotype by replacing the mutated segments with a wild-type one and restoring both superhelicity and motility.