THE DELTA-CRYSTALLIN ENHANCER-BINDING PROTEIN DELTA-EF1 IS A REPRESSOR OF E2-BOX-MEDIATED GENE ACTIVATION

The repressor delta EF1 was discovered by its action on the DC5 fragme nt of the lens-specific delta 1-crystallin enhancer. C-proximal zinc f ingers of delta EF1 were found responsible for binding to the DC5 frag ment and had specificity to CACCT as revealed by selection of high-aff inity binding sequences from a random oligonucleotide pool. CACCT is p resent 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 delta EF1 accounts for the E2 box repressor activity. delta EF1 competed with E47 for binding to an E2 box sequence in vitro. In lymph oid cells, endogenous delta EF1 activity as a repressor was detectable , and exogenous delta EF1 repressed immunoglobulin K enhancer by bindi ng to the kappa E2 site. Moreover, delta EF1 repressed MyoD-dependent activation of the muscle creatine kinase enhancer and MyoD-induced myo genesis of 10T1/2 cells. Thus, delta EF1 counteracts basic helis-loop- helix activators through binding site competition and fulfills the con ditions of the E2 bos repressor. In embryonic tissues, the most promin ent site of delta EF1 expression is the myotome. Myotomal expression a s well as the above results argues for a significant contribution of d elta EF1 in regulation of embryonic myogenesis through the modulation of the actions of MyoD family proteins.