Figure 9

Cartoon figure of zebrafish six3a regulation by modules A and D. The black line represents the six3a genomic DNA, and the blue boxes represent the functional and evolutionarily conserved elements identified in this study. There are four elements in module A and three in module D. Each module is occupied by predictive transcription factors, some of which are activators (circles), and others are repressors (triangles). The 28-bp element in module D is bound by the POU and Gbx1 transcription factors, enhancing expression by interacting with the basal promoter. Elements #4 and #5 in module D, presumptively occupied by Pax6.1 and Foxa2, also enhance six3a expression by interacting with the basal promoter. There are four elements in module A, none of which can function independently. Through the interaction between POU and Pax6.1 and the 1^st to 3^rd elements, module A enhances six3a expression by interacting with the basal promoter. To inhibit the ectopic expression of six3a, zfoxl1 binds to module A through the 2^nd element and module D through the #5 element, possibly eliminating the Foxa2 interaction with the same DNA. Alternatively, LMO4b may bind to the 4^th element in module A and simultaneously to the 28-bp element in module D, perhaps eliminating Gbx1 binding to the same DNA. The repression effect can be achieved by module A alone or with both modules together.