max encodes a sequence-specific DNA-binding protein and is not regulated by serum growth factors

Max and c-Myc proteins, produced in bacteria, were studied for DNA-binding activity using the electrophoretic band-shift assay (EMSA). Both Max homodimers and c-Myc-Max heterodimers selected the same sequence CA(C/T)GTG from an initial pool of 10⁶ DNA molecules. From the pool of sequence-specific binding sites, the palindromic site (CACGTG) was preferentially selected over the CATGTG site using two different degenerate oligonucleotide probes, max expression is identical in myc-induced tumor cell lines relative to other cells. Furthermore, max expression is constant in both confluent and serum-stimulated A31 fibroblasts, in contrast to c-myc expression, which is barely detectable in confluent fibroblasts and induced 20-fold by serum growth factors. Based on recognition of the same DNA sequence by Max and c-Myc-Max complexes and differential expression of the two genes, we propose that Max homodimers and c-Myc-Max heterodimers may bind to a common set of cellular target genes.