Differential patterns of DNA binding by myc and max proteins.

c-myc, N-myc, and L-myc genes are subject to highly variable degrees of tissue-specific regulation. Their aberrant expression has also been implicated in the pathogenesis of a variety of malignant tumors. The recently identified max protein dimerizes with c-myc to promote its sequence-specific DNA binding. max exists in two forms (long and short) that differ by virtue of a 9-amino acid insertion/deletion at the N terminus. We tested recombinant myc and max proteins for binding to six oligonucleotides containing related c-myc sites. Each myc protein, alone and in association with max proteins, manifested a unique pattern of DNA binding. Phosphorylation of both max proteins was observed when they were incubated in a rabbit reticulocyte lysate. This strongly affected DNA binding by max(long) but not by max(short). Our results point to the existence of specific DNA binding preferences for each of the myc proteins. The 9-amino acid segment that distinguishes max(long) from max(short) appears to serve a regulatory function that provides additional control over DNA sequence recognition.