THE HMG DOMAIN PROTEIN SSRP1 PREIIBF IS INVOLVED IN ACTIVATION OF THEHUMAN EMBRYONIC BETA-LIKE GLOBIN GENE/

The human embryonic beta-like globin (epsilon-globin) gene is expresse d in primitive erythroid cells of the yolk sac during the first few we eks of development. We have previously shown that developmental stage- specific expression of the epsilon-globin gene is mediated by multiple positive and negative regulatory elements upstream of the start of tr anscription. Of particular interest is one positive regulatory element , PRE II, that works together with other elements (PRE I and PRE V) to confer developmental stage- and/or tissue-specific expression on a mi nimal promoter. An similar to 85- to 90-kDa PRE II binding factor (PRE IIBF) was identified in the nuclei of erythroid cells and shown to bin d specifically to a novel 19-bp region within PRE II; binding of this protein to PRE II resulted in bending of the target DNA and was requir ed for promoter activation. In this report, we present the cDNA expres sion cloning of PREIIBF. The cDNA encodes a previously identified memb er of the HMG domain family of DNA binding proteins termed SSRP1. By a number of biochemical and immunological criteria, recombinant SSRP1 a ppears to be identical to the PREII binding factor from erythroid nucl ei. A hallmark of HMG domain proteins is their ability to bend their t arget DNAs; therefore, as we speculated previously, DNA bending by SSR P1/PREIIBF may contribute to the mechanism by which PRE II synergizes with other regulatory elements located upstream and downstream. In con trast with reports from other investigators, we demonstrate that SSRP1 binds DNA with clear sequence specificity. Moreover, we show that SSR P1/PREIIBF lacks a classical activation domain but that binding by thi s protein to PRE II is required for activation of a minimal promoter i n stable erythroid cell lines. These studies provide the first evidenc e that SSRP1 plays a role in transcriptional regulation. SSRP1/PREIIBF may serve an architectural function by helping to coordinate the asse mbly of a multiprotein complex required for stage-specific regulation of the human epsilon-globin gene.