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Fig. 7. Schematic representation of the different cis-acting elements
controlling Krox20 expression in the hindbrain and derived neural
crest. Within the Krox20 chick and mouse syntenic regions, the
different cis-acting elements show an identical general organisation, although
the distances relative to the Krox20 gene vary between species. In
this scheme, elements B and C (in orange) are responsible for the initiation
of Krox20 expression in r3 and r5, with a likely redundancy in r5,
where vHNF1 participates in the activation of element B. This leads to
accumulation of Krox20 that then activates element A (in blue), which is
responsible for the maintenance of Krox20 expression in the
neuroepithelium by positive autoregulation. In addition, the combined action
of elements A, B and C is responsible for reaching a threshold level of Krox20
protein in the dorsal part of r5, which together with Sox10 initiates another
positive autoregulatory loop, the latter involving the NCE element (in green),
that maintains Krox20 expression in migrating r5-derived neural crest
cells (Ghislain et al.,
2003).
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