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Three binding proteins, one specific for guanosine 3′: 5′-monophosphate (cGMP) and two specific for adenosine 3′: 5′-monophosphate (CAMP) have been partially purified from vegetative cells of Myxococcus xanthus M300. The cGMP binding activity was found in the periplasmic shock fluid. Scatchard analysis indicated only a single class of binding sites with high affinity (apparent K D, 42 nm). The two cAMP binding activities were physically distinct, as indicated by their elution patterns from DEAE-cellulose, K D values and cellular locations. The cytoplasmic cAMP binding protein, which is probably identical to that previously isolated from developing myxospores of M. xanthus had an apparent K D of 57 nm, whereas the periplasmic cAMP binding protein had an apparent KD of 1 μm. During development, the nucleotide binding proteins exhibited changes in activities consistent with their postulated roles during fruiting body development.