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 expre ssion of the segment polarity genes wingless, engrailed and gooseberry , It encodes a protein containing three conserved motifs: a homeodomai n (HD), a paired domain (PD) and a PRD (His/Pro) repeat, We use a resc ue assay in which paired (or a mutated version of paired in which the functions of the conserved motifs have been altered) is expressed unde r 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 require d within the same molecule for normal paired function, In contrast, th e conserved C-terminal subdomain of the PD (RED domain) appears to be dispensable, Furthermore, although a mutation abolishing the ability o f 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 imp aired, but still able to rescue to viability, We conclude that, while Prd can use its DNA-binding domains combinatorially in order to achiev e different DNA-binding specificities, its principal binding mode requ ires a cooperative interaction between the PAI domain and the homeodom ain.