Model for the differential regulation of
runt expression by pair-rule proteins at late phase 2 (
A,
B)
versus early phase 3 (
C,
D). (
A,
C) Schematic diagrams showing the expression of
runt relative to potential regulators. The horizontal axis represents an idealised portion of the AP axis (~12 nuclei across). The grey vertical lines demarcate a double parasegment repeat (~8 nuclei across). Lighter red and green sections in (
C) represent fading Eve and Runt protein. (
B,
D) Inferred regulatory interactions. Separate regulatory logic is shown for the expression of the primary (1ary) stripes and the secondary (2ary) stripes, because they are driven by separate enhancers (see text for details). Hammerhead arrows represent repressive interactions. Solid black arrows represent interactions that are currently in operation; dashed grey arrows represent those that are not. At each stage,
runt is expressed only where its current repressors are absent. See
Appendix 1—figure 3 for staged relative expression data. Note that the expression patterns of potential regulators diagrammed in this figure represent protein distributions, which often differ slightly from transcript distributions due to time delays inherent in protein synthesis and decay (see
Figure 6).