2 HOMEO DOMAIN PROTEINS BIND WITH SIMILAR SPECIFICITY TO A WIDE-RANGE OF DNA SITES IN DROSOPHILA EMBRYOS

We have used in vivo UV cross-linking to directly measure DNA binding by the homeo domain proteins even-skipped (eve) and fushi tarazu (ftz) in Drosophila embryos. Strikingly, these two proteins bind at uniformly high levels throughout the length of their genetically identified target genes and at lower, but significant, levels to genes that they are not expected to regulate. The data also suggest that these two proteins have very similar DNA-binding specificities in vivo. In contrast, a non-homeo domain transcription factor, zeste, is only detected on short DNA elements within a target promoter and not on other genes. These results are consistent with the in vitro properties of these various proteins, their respective concentrations in the nucleus, and with earlier predictions of how transcription factors bind DNA in vivo. We propose that these data favor the model that eve, ftz, and closely related homeo domain proteins act by directly regulating mostly the same target genes.