Distinct factors bind to apparently homolgous sequences in the immunoglobulin heavy-chain enhancer

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Abstract
The intron separating the variable- and constant-region exons of the rearranged immunoglobulin heavy-chain locus contains a lymphocyte-specific transcriptional enhancer1,2. The enhancer is a member of a class of cis-acting, tissue-specific, transcriptional control elements which are characterized by orientation-independent and relatively position-independent function1,2. In vivo analysis of the position of DNA-binding factors, by assessing the availability of specific bases to chemical modification, has identified four sequence clusters within the heavy-chain enhancer, denoted El to E4 (refs 3, 4). These sites are protected (that is, occupied) only in B lymphocytes. A consensus sequence relationship (consensus CAGGTGGC) between these four sites was suggested where three of the sites conformed to the consensus in seven of eight positions while the other was homolgous in six of eight positions. We proposed that a single trans-acting factor might recognize all four sites3,4. Using an assay involving gel electro-phoresis of DNA–protein complexes5–8 to detect sequence-specific DNA binding factors that recognize these related motifs, we have now identified a mouse B-cell nuclear factor (NF-μE1) which binds specifically to one such motif within the mouse heavy-chain gene enhancer. This factor binds poorly, if at all, to the other related motifs, and other factors have been identified which interact preferentially with some of these latter motifs. Dimethyl sulphate interference experiments suggest that the NF-μE1 factor is in contact with at least the guanine residues in the seqeunce GATGGCCGATC. This factor seems to be present in both lym-phoid and non-lymphoid cell lines.