MODULAR ORGANIZATION OF PAX HOMEODOMAIN PROTEINS IN TRANSCRIPTIONAL REGULATION/

Specificity in transcriptional regulation lies in a large part in the specificity of DNA binding by transcription factors. One group of tran scription factors which are of great interest for studying transcripti onal specificity is the Pax/Homeodomain (Pax/HD) proteins which contai n two conserved DNA binding domains, a paired domain (PD) and a Paired -class homeodomain (HD). The Pax/HD proteins can bind to at least thre e types of specific DNA sequences: the PD binding sites, the dimeric H D binding sites and a composite HD and PD binding site. We propose tha t Pax/HO proteins regulate different subsets of their target genes thr ough modular binding to one of these three specific sequences. We show that, in a tissue culture system, a member of the Pax/HD family, Pair ed, is able to activate transcription after binding through either its PD or its HD. The transactivation mediated by one domain does not req uire DNA binding of the other domain. Furthermore, binding sites speci fic for the PD of Paired are sufficient to mediate embryonic expressio n of a reporter gene in a paired-like pattern. The expression of the r eporter gene is dependent on wild type paired function and, in a prd m utant background, it can be rescued by an exogenous paired gene encodi ng a protein whose HD is not able to bind to DNA. Finally, we show tha t the Paired protein uses differently its C-terminal activation domain when transactivation is mediated through its PD or its HD. These resu lts and recent evidence from other Pax/HD proteins strongly suggest th at this class of proteins is able to achieve specific and modular tran scriptional regulation through its multiple DNA binding domains.