Protein kinase C normally has a tandem repeat of a characteristic cysteine-rich sequence in C1, the conserved region of the regulatory domain. These sequences resemble the DNA-binding zinc finger domain. For the gamma subspecies of rat brain protein kinase C, various deletion and point mutants in this domain were constructed, and the mutated proteins were expressed in Escherichia coli by using the T7 expression system. Radioactive phorbol 12,13-dibutyrate binding analysis indicated that a cysteine-rich zinc-finger-like sequence was essential for protein kinase C to bind phorbol ester and that one of two sequences was sufficient for the phorbol ester binding. Conserved region C2, another region in the regulatory domain, was apparently needed for the enzyme to require Ca2+ for phorbol ester binding activity.