Analysis of c-Myc and Max binding to the c-myc promoter: evidence that autosuppression occurs via an indirect mechanism.

c-myc negatively autoregulates its expression at the level of transcriptional initiation, although the precise mechanism remains debated. While conclusive evidence for c-Myc binding in its own promoter has not been found, it has been proposed that c-Myc binds to a site upstream of the human c-myc gene which may also be a component of a replication origin (Ariga et al., 1989). In an attempt to clarify this issue, sequences flanking the c-myc gene were screened for c-Myc or Max binding sites using a novel procedure to facilitate the detection of DNA binding dependent upon long distance interactions or DNA secondary structure. Since the sequence specificity of DNA binding proteins may also be mediated by interaction with other proteins or by protein modification, this affinity capture assay was used in conjunction with nuclear extracts, potentially allowing the selection of novel in vivo DNA binding specificities. Using conditions that gave strong binding to an internal control sequence, c-Myc or Max binding elements were not detected in genomic sequences extending 5.4 kb upstream of the Xenopus c-myc gene. Identical results were obtained using both purified proteins and a variety of nuclear extracts, suggesting c-myc autosuppression most likely involves an indirect pathway.