Two-step secretion of the Serratia marcescens extracellular nuclease.

The extracellular nuclease of Serratia marcescens is one of a wide variety of enzymes secreted into the growth medium. Its appearance occurs late in the growth of a culture, and its gene, nucA, is transcriptionally regulated in a complex fashion by growth phase and other factors. Pulse-labeling studies reveal that extracellular secretion of nuclease occurs as a two-step process. In the first step, nuclease is rapidly translocated across the cytoplasmic membrane into the periplasm, where it accumulates as a mature active nuclease. A precursor protein, nuclease still carrying its signal sequence, was detected in the presence of carbonyl cyanide m-chlorophenylhydrazone or sodium azide, suggesting that this initial translocation and signal processing step involves an energy-dependent and Sec-dependent pathway in S. marcescens. The second step of secretion across the outer membrane is a slow process requiring between 30 to 120 min, depending on growth conditions.