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Transcriptional activation by the tumor suppressor p53 is regulated at multiple levels, including posttranslational modi®cations of the p53 protein, interaction of p53 with various regulatory proteins, or at the level of sequencespeci®c DNA binding to the response elements in p53's target genes. We here propose as an additional regulatory mechanism that the DNA topology of p53-responsive promoters may determine the interaction of p53 with its target genes. We demonstrate that sequencespeci®c DNA binding (SSDB) and transcriptional activation by p53 of the mdm2 promoter is inhibited when this promoter is present in supercoiled DNA, where it forms a non-B-DNA structure which spans the p53-responsive elements. Relaxation of the supercoiled DNA in vitro resulted in conversion of the non-B-DNA to a B-DNA conformation within the mdm2 promoter, and correlated with an enhanced SSDB of p53 and an elevated expression of a reporter gene. In contrast, sequence speci®c DNA binding and transcriptional activation of the p21 promoter were not inhibited by DNA supercoiling. We propose that conformational alterations within p53-responsive sites, which either promote or prohibit sequence speci®c DNA binding of p53, are an important feature in orchestrating the activation of dierent p53 responsive promoters.

We have identified a zebrafish homolog of the F3/F11/contactin (F3) recognition molecule. The gene shares 55% amino acid identity with F3 molecules in other vertebrates. Expression of F3 mRNA is first detectable at 16 h post-fertilization (hpf) in trigeminal and Rohon-Beard neurons. At 18-24 hpf, additional weaker expression is present in discrete cell clusters in the hindbrain, in the anterior lateral line/acoustic ganglion and in spinal motor neurons. Transcription factors of the LIM homeodomain class (LIM-HD) and their associated cofactors CLIM/NLI/Ldb (CLIM) have been implicated in the development of peripheral axons of trigeminal and Rohon-Beard neurons. We demonstrate that ectopic overexpression of a dominant-negative CLIM molecule early during zebrafish development strongly reduces expression of F3 mRNA in these neurons indicating regulation of F3 by the LIM-HD protein network. These results and the spatiotemporal correlation of F3 expression with axonal differentiation in a subset of primary neurons suggest an important role of F3 for axon growth.

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