Activation of β-Globin Promoter by Erythroid Krüppel-Like Factor

Erythroid Krüppel-like factor (EKLF), an erythroid tissue-specific Krüppel-type zinc finger protein, binds to the β-globin gene CACCC box and is essential for β-globin gene expression. EKLF does not activate the γ gene, the CACCC sequence of which differs from that of the β gene. To test whether the CACCC box sequence difference is the primary determinant of the selective activation of the β gene by EKLF, the CACCC boxes of β and γ genes were swapped and the resulting promoter activities were assayed by transient transfections in CV-1 cells. EKLF activated the β promoter carrying a γ CACCC box at a level comparable to that at which it activated the wild-type β promoter, whereas EKLF failed to activate a γ promoter carrying the β CACCC box, despite the presence of the optimal EKLF binding site. Similar results were obtained in K562 cells. The possibility that overexpressed EKLF superactivated the β promoter carrying the γ CACCC box, or that EKLF activated the mutated β promoter through the intact distal CACCC box, was excluded. To test whether the position of the CACCC box in the β or γ promoter determined EKLF specificity, the proximal β CACCC box sequence was created at the position of the β promoter (−140) which corresponds to the position of the CACCC box on the γ promoter. Similarly, the β CACCC box was created in the position of the γ promoter (−90) corresponding to the position of the CACCC box in the β promoter. EKLF retained weak activation potential on the β^−140CAC promoter, whereas EKLF failed to activate the γ^−90βCAC promoter even though that promoter contained an optimal EKLF binding site at the optimal position. Taken together, our findings indicate that the specificity of the activation of the β promoter by EKLF is determined by the overall structure of the β promoter rather than solely by the sequence of the β gene CACCC box.     

The human gamma-globin TATA and CACCC elements have key, distinct roles in suppressing beta-globin gene expression in embryonic/fetal development

The competition model of globin gene regulation states that the gamma-globin gene precludes expression of the beta-globin gene in early development by competing for the enhancing activity of the locus control region. The gamma-globin gene with a -161 promoter is sufficient for suppressing beta-globin gene expression, and the gamma-globin TATA and CACCC elements are necessary for this effect. In this work, stable transfection and transgenic mouse assays have been performed with constructs containing HS3 and HS2 from the locus control region, the gamma-globin gene with promoter mutation(s), and the beta-globin gene. The data indicate that the gamma-globin TATA and CACCC elements together have at least an additive effect on the beta/gamma-globin mRNA ratio in early erythroid cells, suggesting that the elements work coordinately to suppress beta-globin gene expression. The TATA and CACCC are the major gamma-globin promoter elements responsible for this effect. Transgenic mouse experiments indicate that the gamma-globin TATA element plays a role in gamma-globin expression and beta-globin suppression in the embryo and fetus; in contrast, the CACCC element has a stage-specific effect in the fetus. The results suggest that, as is true for the erythroid Krüppel-like factor (EKLF) and the beta-globin promoter CACCC, a protein(s) binds to the gamma-globin CACCC element to coordinate stage-specific gene expression.