[en] An 11450-base DNA fragment containing the gene for the extracellular active-site serine DD-peptidase of Streptomyces R61 was cloned in Streptomyces lividans using the high-copy-number plasmid pIJ702 as vector. Amplified expression of the excreted enzyme was observed. Producing clones were identified with the help of a specific antiserum directed against the pure DD-peptidase. The coding sequence of the gene was then located by hybridization with a specific nucleotide probe and sub-fragments were obtained from which the nucleotide sequence of the structural gene and the putative promoter and terminator regions were determined. The sequence suggests that the gene codes for a 406-amino-acid protein precursor. When compared with the excreted, mature DD-peptidase, this precursor possesses a cleavable 31-amino-acid N-terminal extension which has the characteristics of a signal peptide, and a cleavable 26-amino-acid C-terminal extension. On the basis of the data of Joris et al. (following paper in this journal), the open reading frame coding for the synthesis of the DD-peptidase was established. Comparison of the primary structure of the Streptomyces R61 DD-peptidase with those of several active-site serine β-lactamases and penicillin-binding proteins of Escherichia coli shows homology in those sequences that comprise the active-site serine residue. When the comparison is broadened to the complete amino acid sequences, significant homology is observed only for the pair Streptomyces R61 DD-peptidase/Escherichia coli ampC β-lactamase (class C). Since the Streptomyces R61 DD-peptidase and β-lactamases of class A have very similar three-dimensional structures [Kelly et al. (1986) Science (Wash. DC) 231, 1429–1431; Samraoui et al. (1986) Nature (Lond.) 320, 378–380], it is concluded that these tertiary features are probably also shared by the β-lactamases of class C, i.e. that the Streptomyces R61 DD-peptidase and the β-lactamases of classes A and C are related in an evolutionary sense.