This paper presents evidence that a polyamine-dependent protein kinase (EC 2.7.1.37) purified from nuclei of the slime mold Physarum polycephalum catalyzes phosphorylation of ornithine decarboxylase (OrnDCase; L-ornithine carboxy-lyase, EC 4.1.1.17). The protein kinase had properties similar to OrnDCase antizyme. Phosphocellulose chromatography of nuclear preparations from P. polycephalum yielded the polyamine-dependent protein kinase of subunit Mr 26,000 that was resolved from a second fraction in which the protein kinase copurified with a phosphate-acceptor protein of subunit Mr 70,000. At Na+ concentrations less than approximately 150 mM, a complex formed between the protein kinase and the phosphate-acceptor protein. The complex did not demonstrate protein kinase or OrnDCase activity. The complex was dissociated by greater than 150 mM Na+ into its constituent proteins. The dissociated complex catalyzed phosphorylation of the Mr 70,000 component in the presence of spermidine and spermine, and it also demonstrated OrnDCase activity. The purified Mr 70,000 component from the complex and authentic OrnDCase, purified by procedures previously reported, were virtually identical with respect to OrnDCase activity, capacity to be phosphorylated by the polyamine-dependent protein kinase, amino acid composition, and immunological crossreactivity. Phosphorylation of OrnDCase by the polyamine-dependent protein kinase sharply inhibited OrnDCase activity. Thus, this is an example of posttranslational covalent modification of OrnDCase with concurrent alteration of its catalytic function. It is also an unusual example of control of the first enzyme in a biosynthetic pathway by a protein kinase that is, in turn, modulated by the immediate end products of the pathway.