Phosphorylation-dependent binding of a 138-kDa myc intron factor to a regulatory element in the first intron of the c-myc gene.

A 138-kDa nuclear protein was identified from HeLa cell extracts as a factor which binds to a previously described 20-base pair cis element located in the intron I of the c-myc gene. This myc intron factor (MIF) binds to the wild type c-myc sequence but does not bind under similar conditions to c-myc from Burkitt's lymphoma which contain point mutations in this binding region. We have demonstrated that the 138-kDa MIF is a phosphoprotein and that treatment of the purified MIF with potato acid phosphatase abolished binding to its 20-base pair c-myc recognition sequence; binding activity was protected by inclusion of phosphatase inhibitors. These results suggest that phosphorylation is required for the specific DNA-MIF interaction in vitro and that the phosphorylation state of MIF may be an important factor in controlling c-myc expression in vivo.