Developmental modulation of protein binding to beta-globin gene regulatory sites within chicken erythrocyte nuclei.

Overview

We describe the interaction of two adjacent binding sites in the chicken beta-globin gene promoter with regulatory factors present in erythroid cells. One of these sites is a palindromic sequence (Pal) that binds a member of the nuclear factor 1 family; the other is the CACCC sequence found in most adult beta-globin promoters. Transfection of primary erythrocytes with plasmids carrying the gene coupled to truncated promoters reveals that the Pal site inhibits and the CACCC site stimulates expression. Nuclease protection experiments on intact nuclei show that at early stages of embryonic development, the CACCC site is occupied and the Pal site is vacant, but as development progresses, the Pal site is filled gradually and the CACCC site loses its bound protein. Beyond day 15 of development, Pal is completely occupied and CACCC is empty in vivo. Parallel DNase I footprinting and gel retardation studies in vitro show that nuclear extracts contain sharply increasing Pal-binding activity as development proceeds, but CACCC-binding activity falls off only slightly. We show that the two factors bind to their sites in vitro in an anticooperative manner and conclude that this could account for the observed changes in site occupancy in vivo. Our results suggest that the Pal factor may play a role in the shutdown of adult beta-globin expression late in erythroid development.