Methylation of the mouse M-lysozyme downstream enhancer inhibits heterotetrameric GABP binding

Expression of the mouse M-lysozyme gene is a specific marker for the differentiation of macrophage/granulocyte cell lineages. Analysis of the mechanisms regulating M-lysozyme gene expression revealed an enhancer element in the 3'-flanking region of the gene, termed the M-lysozyme downstream enhancer (MLDE). Here we demonstrate that the nuclear factors binding to MLDE are present in all tested myeloid and non-myeloid mouse cell lines. Sequence analysis of MLDE identified two different sequences, CAGGAAGT and CCGGAAGT, which match the consensus binding sequences for proteins of the ets gene superfamily. The two sites are oriented palindromicly and separated by 10 bp. DMS/DEPC interference assays revealed different patterns of DNA-protein contacts on the two sites. Mutation of each consensus sequence leads to an individual change in protein binding in vitro. Despite these differences, both sequences are bound by GABP, forming a heterotetrameric complex. Tissue specificity is correlated with demethylation of a single CPG dinucleotide located in one of the two Ets motifs. This site when methylated inhibits GABP binding to both sequences in non-macrophage cell types.