ISOFORMS OF SERRATIA-MARCESCENS NUCLEASE PRODUCED BY NATIVE AND RECOMBINANT STRAINS - COMPARATIVE CHARACTERIZATION BY PLASMA DESORPTION MASS-SPECTROMETRY

The primary structures of isoforms of native nuclease secreted by Serr atia marcescens and of recombinant nuclease produced by Escherichia co il have been characterized by plasma desorption mass spectrometry. The isoforms were isolated from the purified nuclease on a DEAE-cellulose anion-exchange column and then digested with the endoproteinase Lys-C . The resulting peptides were isolated by reverse phase HPLC and their molecular weights determined by plasma desorption mass spectrometry. Among the nuclease isoforms secreted by the parent cells, a new isofor m Sm3 was found. Comparison of the peptides produced from nuclease Sm2 and two other isoenzymes (Sm1 and Sm3) showed that they differed only in the N-termini; three amino acid residues are lacking in Sm1 and on e residue in Sm3. Identity was demonstrated between nucleases Sm1 and Sm2 produced by S. marcescens and the isoforms rSm1 and rSm2 secreted by the recombinant E. coli strain.