COVER: Vibrio alginolyticus has only a single fl agellum at the cell pole. Its place and number are regulated by the two key factors, GTPase FlhF which is a homolog of the secretion factor FtsY and ATPase FlhG which is a paralogue of the cell division regulator MinD. The GTPase activity of FlhF is activated by FlhG, which is localized at the pole by the HubP scaffold. The polar fl agellar genes of Vibrio form a regulon consisting of four hierarchies, from class 1 to class 4. Expression of the class 2 gene group is controlled by FlaK, which functions as a master regulator and belongs to the class 1. flaL and flaM are class 2 genes and regulate expression of class 3 genes. FlhG inhibits FlaK and thereby class 2 gene transcription in the presence of cyclic diguanylate (c-di-GMP). FlhG defi ciency increases fl agellar gene expression and fl agellar number. Homma M., Mizuno A., Hao Y. and Kojima S. (2022) Functional analysis of the N-terminal region of Vibrio FlhG, a MinD-type ATPase in flagellar number control. J. Biochem. 172, 99-107