Site-specific mutagenesis of maz revealed that the serine residue at position 480 was the major site of phosphorylation by ckii both in vitro and in vivo. Phosphorylation of maz by ckii at this serine residue was required for maximum binding of maz to the pyrimidine-rich dna of the nuclease-hypersensitive element (nhe) in the 5'-end promoter region of the c-myc gene. Mutation of serine at position 480 to alanine eliminated the dna-binding activity of maz to this element.