DIFFERENTIAL SECRETION OF ISOFORMS OF SERRATIA-MARCESCENS EXTRACELLULAR NUCLEASE

Extracellular secretion of the Serratia marcescens nuclease occurs in a two-step process: (i) rapidly to the periplasm via a signal sequence -dependent pathway and then (ii) slowly to the extracellular growth me dium without cell lysis. There are two major isoforms of the nuclease in the culture supernatant of S. marcescens. We have isolated, purifie d, and determined the sequences of both isoforms, The first isoform, t he mature nuclease (Sm2), is the result of signal sequence processing, The second isoform (Sm1) has three additional amino acids missing fro m the N terminus of the mature nuclease. Sm1 starts to appear extracel lularly only during prolonged growth of a culture (16 to 48 h), probab ly because of cell lysis, However, pulse-chase experiments show that i t is made early with Sm2 but is not secreted efficiently.