RECOGNITION OF A DNA OPERATOR BY A DIMER COMPOSED OF 2 DIFFERENT HOMEODOMAIN PROTEINS

The yeast homeodomain proteins a1 and alpha2 interact to form a heterodimer that binds DNA with high specificity. The DNA recognition element consists of two similar half-sites, arranged with dyad symmetry and separated by a fixed number of base pairs. We demonstrate that in the a1alpha2 - DNA complex, one of these half-sites is bound by al while the other is bound by alpha2. These assignments allow a comparison of the chemical and nuclease protection patterns produced by both proteins when bound together to the hsg operator. Contrary to simple expectations, we propose that the a1 and alpha2 homeodomains are arranged on the DNA in tandem, despite the fact that the recognition sequence is dyad symmetric.