A model for spatio-temporal <i>bab2</i> regulation during distal limb development.

<p>(A) LAE functional organization. Relevant <i>cis</i>-regulatory features are delineated beneath the DNA stretch. Within the 5′-half, critical for distal-limb specificity, key regions required for ring activation and inter-ring repression are shown as green and red lines, respectively. The 3′-half, which is only required for high level of expression (signal amplifier or “booster”), is depicted as an orange line. The binding sites for the direct <i>bab2</i> regulators Dll and Rn (in grey and light green, respectively) as well as for an unknown factor (X, in dark green) required for normal distal-most ring expression level, are indicated by vertical arrows. (B) A model for spatio-temporal regulation of <i>bab2</i> gene during third-instar larval (L3) stage in antennal (ant) and leg imaginal discs (see main text for details).