The δ-crystallin enhancer-binding protein δEF1 is a repressor of E2- box-mediated gene activation

The repressor δEF1 was discovered by its action on the DC5 fragment of the lens-specific δ1-crystallin enhancer. C-proximal zinc fingers of δEF1 were found responsible for binding to the DC5 fragment and had specificity to CACCT as revealed by selection of high-affinity binding sequences from a random oligonucleotide pool. CACCT is present not only in DC5 but also in the E2 box (CACCTG) elements which are the binding sites of various basic helix- loop-helix activators and also the target of an unidentified repressor, raising the possibility that δEF1 accounts for the E2 box repressor activity. δEF1 competed with E47 for binding to an E2 box sequence in vitro. In lymphoid cells, endogenous δEF1 activity as a repressor was detectable, and exogenous δEF1 repressed immunoglobulin κ enhancer by binding to the κE2 site. Moreover, δEF1 repressed MyoD-dependent activation of the muscle creatine kinase enhancer and MyoD-induced myogenesis of 10T1/2 cells. Thus, δEF1 counteracts basic helix-loop-helix activators through binding site competition and fulfills the conditions of the E2 box repressor. In embryonic tissues, the most prominent site of δEF1 expression is the myotome. Myotomal expression as well as the above result argues for a significant contribution of δEF1 in regulation of embryonic myogenesis through the modulation of the actions of MyoD family proteins.