Ion channels gated by cyclic nucleotides have crucial roles in neuronal excitability and signal transduction of sensory neurons. Here, we studied ligand binding of a cyclic nucleotide-activated K(+) channel from Mesorhizobium loti and its isolated cyclic nucleotide-binding domain. The channel and the binding domain alone bind cyclic AMP with similar affinity in a non-cooperative manner. The cAMP sensitivities of binding and activation coincide. Thus, each subunit in the tetrameric channel acts independently of the others. The binding and gating properties of the bacterial channel are distinctively different from those of eukaryotic cyclic nucleotide-gated channels.