Figure 7.

Schematic representation of the possible crossregulatory interactions between Krox20 and PIASxβ. The relative levels of the two proteins determine the expression of r3/r5- and r4-specific genes (see the Discussion section for details of the model). The circles represent the different proteins and the rectangle the cis-acting elements. (A) Situation in r3/r5. (B) Situation in r4. The interaction of Piasxβ with the Hoxb1 locus may be direct or indirect, via a putative protein (X). The domain of PIASxβ involved in this latter interaction has not been characterized. It is arbitrarily represented as the domain overlapping with the Krox20-binding domain. This representation does not take into account the possibility that PIASxβ might be acting by sequestering a repressor of Hoxb1.