ISOLATION AND CHARACTERIZATION OF ISOFORMS OF THE EXTRACELLULAR NUCLEASE OF SERRATIA-MARCESCENS

Two enzyme forms (nucleases Sm1 and Sm2), differing in chromatographic and electrophoretic properties, isolectric points, and N-terminal amino acids, were isolated from a commercial preparation of the extracellular nuclease of Serratia marcescens strain B10 M1. The yield of the nucleases Sm1 and Sm2 in a two-step purification procedure, using ion exchange chromatography on phospho- and DEAE-celluloses, was 13 and 25%, respectively. The nucleases are close in specific activity, equal to 3.6 . 10(6) and 4.6 . 10(6) activity units per mg protein for nucleases Sm1 and Sm2. A comparative analysis was made of the peptides of tryptic hydrolysates of nucleases, and the amino acid sequences of individual portions of the polypeptide chains of the proteins were determined. High similarity of the structures was demonstrated, and a local difference of the amino acid sequences in the N-terminal segments of the enzymes was detected. The positions of the disulfide bonds in the nuclease molecules were traced. It is concluded that the isoforms of the nucleases Sm1 and Sm2 of the strain B10 M1 are similar to the endonucleases of Serratia marcescens isolated from other strains of bacteria.