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Fig. 7. A model for the combinatorial coding of DA3 muscle identity in
Drosophila. (A) Col is activated in one (T1-T3 segments)
and two (A1-A2 segments) promuscular clusters
(Crozatier and Vincent, 1999),
in response to positional and mesodermal cues. This first step is probably
mediated by clusters of relevant TF-binding sites [light orange boxes
(Philippakis et al., 2006)],
including Twi-binding sites (+) (Sandmann
et al., 2007) that are located within the col upstream
region and introns. Col expression subsequently becomes restricted to the
DA3/DO5 progenitor (orange cell) by lateral inhibition
(Crozatier and Vincent, 1999).
We postulate that positive inputs from TFs binding to the -2.6 to -2.3
fragment, including Twi, are sufficient to allow P2.6cl
activation in the selected DA3/DO5 progenitor, upon relief of N repression.
(B) Following division of the progenitor, restriction of Col expression
to the DA3 FC involves positive auto-regulation in this FC and negative
regulation by N in the sibling DO5 FC. From this stage, a combination of Nau
and Col activity is required for col transcriptional activation in
the FCM nuclei, which are recruited by the DA3 FC to form a myofibre, thereby
ensuring that all nuclei in the DA3 muscle express the same identity
programme.
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