Mystery of fliK in length control of the flagellar hook.

Along history of the long hook: the first paper regarding hook length control in the flagellum appeared about 40 years ago. Silverman and Simon (1972) found that a flaE (the previous name of fliK in Escherichia coli) mutant produced extraordinarily long hooks, which they named polyhooks, and concluded that the flaE gene “functions to control the length of the hook” (22). Around the same time, Patterson-Delafield et al. (1973) found that a flaR (the previous name of fliK in Salmonella enterica serovar Typhimurium) mutant similarly produced extraordinarily long hooks, which they named superhooks (20). “Polyhook” connotes a repeated end-to-end polymer of the hook, whereas “superhook” simply connotes a long hook. Retrospectively, “superhook” might have been the proper name, since there is no evidence so far to show that the lengths of long hooks occur as integral multiples of the wild-type hook length. But we respect history and call it a polyhook today. Ironically, later on, we found extraordinary long polyhooks and named them super-polyhooks (19). Because this topic has a long history and has involved contradictive explanations of similar data from many research groups, it is not easy to present all experimental data in chronological order. Many ideas and models have come and gone, but none of them is fully satisfactory. In this article, I discuss the currently most popular model from other laboratories and present our new model, which naturally opposes the current model. Because of limited space, I have to skip most of the biochemical and genetic data (Kelly Hughes may discuss these aspects in his paper) (7, 8) and also skip studies done with the needle complex of pathogenic bacteria (I see more similarities than differences between the two systems). Instead, I focus on physical aspects of the hook length control and begin with the statistics of hook length.