Drosophila homologs of transcriptional mediator complex subunits are required for adult cell and segment identity specification

The origins of specificity in gene expression are a central concern in unde rstanding developmental control. Mediator protein complexes regulate transc riptional initiation, acting as modular adaptors linking specific transcrip tion factors to core RNA polymerase II. Here, we identified the Drosophila homologs of 23 human mediator genes and mutations of two, dTRAP240 and of d TRAP80 (the putative fly homolog of yeast SRB4). Clonal analysis indicates a general role for dTRAP80 necessary for cell viability. The dTRAP240 gene is also essential, but cells lacking its function are viable and proliferat e normally. Clones reveal localized developmental activities including a se x comb cell identity function. This contrasts with the ubiquitous nuclear a ccumulation of dTRAP240 protein in imaginal discs. Synergistic genetic inte ractions support shared developmental cell and segment identity functions o f dTRAP240 and dTRAP80, potentially within a common complex. Further, they identify the homeotic Sex combs reduced product, required for the same cell /tissue identities, as a functional partner of these mediator proteins.