Bacterial CMP kinases are specific for CMP and dCMP, whereas the rela-ted eukaryotic NMP kinase phosphorylates CMP and UMP with similar efficiency. To explain these differences in structural terms, we investigated the contribution of four key amino acids interacting with the pyrimidine ring of CMP (Ser36, Asp132, Arg110 and Arg188) to the stability, catalysis and substrate specificity ofEscherichia coliCMP kinase.