CHARACTERIZATION OF SERRATIA-MARCESCENS NUCLEASE ISOFORMS BY ELECTROSPRAY MASS-SPECTROMETRY

Isoforms of a nuclease were isolated from the culture fluid of S. marc escens B10M1, and their molecular masses were determined by electrospr ay mass spectrometry. The primary structure of each of the isoforms is olated from the nuclease preparations was determined by comparing its molecular mass with that calculated for the known amino acid sequence of the protein encoded by nuc-gene. The structure of the majority of i soforms was verified by isoelectric focusing, and, in a number of case s, also by the N-terminal amino acid sequencing. All isoforms were fou nd to be variants of the ''mature'' form of the nuclease and arise fro m the cleavage of the N-terminus of the protein.