In vivo requirement for the paired domain and homeodomain of the paired segmentation gene product.

The Drosophila pair-rule gene paired is required for the correct expression of the segment polarity genes wingless, engrailed and gooseberry. It encodes a protein containing three conserved motifs: a homeodomain (HD), a paired domain (PD) and a PRD (His/Pro) repeat. We use a rescue assay in which paired (or a mutated version of paired in which the functions of the conserved motifs have been altered) is expressed under the control of its own promoter, in the absence of endogenous paired, to dissect the Paired protein in vivo. We show that both the HD and the N- terminal subdomain of the PD (PAI domain) are absolutely required within the same molecule for normal paired function. In contrast, the conserved C-terminal subdomain of the PD (RED domain) appears to be dispensable. Furthermore, although a mutation abolishing the ability of the homeodomain to dimerize results in an impaired Paired molecule, this molecule is nonetheless able to mediate a high degree of rescue. Finally, a paired transgene lacking the PRD repeat is functionally impaired, but still able to rescue to viability. We conclude that, while Prd can use its DNA-binding domains combinatorially in order to achieve different DNA-binding specificities, its principal binding mode requires a cooperative interaction between the PAI domain and the homeodomain.