AbstractThe cAMP‐dependent protein kinase (cAK) fromDictyostelium discoideumis an enzyme composed of one catalytic and one regulatory subunit. Upon binding of cAMP, the holoenzyme dissociates to liberate free active catalytic subunits. The cAK is developmentally regulated, ranging from very little activity in vegetative cells to maximal expression in postaggregative cells. Although there is no immunological cross‐reaction between the subunits of cAKs fromDictyosteliumand from other organisms, they share several biochemical properties. A complete cDNA for the regulatory subunit has been cloned and sequenced. Only one copy of the gene for the regulatory subunit is present per haploid genome. On the basis of the comparison of the structure of the cAK fromDictyosteliumwith its counterparts in yeast and higher eukaryotes, we propose a model for the evolution of cyclic‐nucleotide‐binding p