FIGURE 5.

Nab factors antagonize Krox20-activating functions. Chick embryo neural tubes were electroporated on the left side with the expression constructs indicated above each column, in situ hybridized with the probes indicated on the left, and flat mounted. A-H, Krox20 is repressed following both mouse Nab1 and Nab2 overexpression (A and E; the arrowheads point to regions where repression is the strongest). Nab is also repressed by Nab1 (B) but not significantly by Nab2 (F). In contrast, EphA4 is only weakly repressed by Nab1 (C, arrowhead) but more efficiently by Nab2 (G, arrowhead). Hoxb1 expression is not affected by Nab1 or Nab2 (D and H). Note that the repression is mostly observed in the dorsal part of the neural tube, reflecting a higher efficiency of electroporation, as observed previously (37). Similar effects are observed in cases of activation (see I, for instance. I-L, the Krox20 I268F protein very efficiently activates Krox20, Nab, and EphA4 and represses Hoxb1 like the wild type protein. Anterior is up. r, rhombomere.