Sox2 is universally expressed in the neural and placodal primordia in early stage embryos, and this expression depends on various phylogenetically conserved enhancers having different regional and temporal specificities. The enhancer N‐3 was identified as a regulator of the Sox2 gene active in the diencephalon, optic vesicle, and after the contact of the vesicle with the ectoderm, in the lens placodal surface area, suggesting its involvement in embryonic visual system development. A 36‐bp minimal essential core sequence was defined in the 568‐bp‐long enhancer N‐3, which in a tetrameric form emulates the original enhancer activity. The core sequence comprises a SOX‐binding sequence and a non‐canonical PAX6 (Paired domain) binding sequence, and is activated by the synergistic action of SOX2 and PAX6 in transfected cells. The SOX and PAX6 binding sequences of the N‐3 core are arranged with the same orientation and spacing as the DC5 sequence of the δ‐crystallin enhancer previously demonstrated to be cooperatively bound by SOX2 and PAX6. The N‐3 core sequence was also bound by these factors in a cooperative fashion, but with a higher threshold of these factors’ levels than DC5, and the enhancer effect of the tetrameric sequence activated by exogenous SOX2 and PAX6 was less pronounced than that of DC5. The observations suggest that gene activation mechanisms that depend on the cooperative interaction of SOX2 and PAX6 but with different thresholds of the factor levels are crucial for the regulation of visual system development.

The bacterial flagellar hook is a tubular helical structure made by the polymerization of multiple copies of a protein, FlgE. Here we report the structure of the hook fromCampylobacter jejuniby cryo-electron microscopy at a resolution of 3.5 Å. On the basis of this structure, we show that the hook is stabilized by intricate inter-molecular interactions between FlgE molecules. Extra domains in FlgE, found only inCampylobacterand in related bacteria, bring more stability and robustness to the hook. Functional experiments suggest thatCampylobacterrequires an unusually strong hook to swim without its flagella being torn off. This structure reveals details of the quaternary organization of the hook that consists of 11 protofilaments. Previous study of the flagellar filament ofCampylobacterby electron microscopy showed its quaternary structure made of seven protofilaments. Therefore, this study puts in evidence the difference between the quaternary structures of a bacterial filament and its hook.