THE INTERACTION WITH DNA OF WILD-TYPE AND MUTANT FUSHI-TARAZU HOMEODOMAINS

The in vitro DNA binding properties of wild-type and mutant fushi tarazu homeodomains (ftz HD) have been analysed. The DNA binding properties of the ftz HD are very similar to those of the Antp HD. In interference experiments with mutant ftz HDs, close approaches between specific portions of the ftz HD peptide and specific regions of the binding site DNA were mapped. A methylation interference, G7 on the β strand of BS2, is absent from the interference pattern with a mutant ftz HD [ftz (R43A) HD] in which the Arg43 at the second position of helix III (the recognition helix) is replaced by an Ala. This indicated that Arg43 of the ftz HD is in close proximity to the N7 of G7 of the β strand of BS2 in the major groove. The methylation and ethylation interference patterns with the ftz (NTD) HD, in which the first six amino acids of the homeodomain were deleted, were extensively altered relative to the ftz HD patterns. Methylation of A11 and G12 of the α strand and ethylation of the phosphate of nucleotide A12 of the α strand no longer interfere with binding. This indicated that the first six amino acids of the homeodomain of ftz interact with A11 of the α strand in the minor groove, the phosphate of the nucleotide A13 on the α strand and G12 of the α strand in the adjacent major groove of BS2. In a binding study using a change of specificity mutation [ftz (Q50K) HD], in which the Gln50 at the ninth position of the third helix is exchanged for a Lys (as in the bicoid HD), and variant binding sites, we concluded that position 50 of the ftz HD and the ftz (Q50K) HD peptides interacts with base pairs at positions 6 and 7 of BS2. These three points of contact allowed us to propose a crude orientation of the ftz HD within the protein-DNA complex. We find that the ftz HD and the Antp HD peptides contact DNA in a similar way.