Serratia marcescens, a gram-negative enteric bacterium, is capable of secreting a number of proteins extraceliularly. The types of activity found in the growth media include proteases, chitinases, a nuclease, and a lipase. Genetic studies have been undertaken to investigate the mechanisms used for the extracellular secretion of these exoproteins by S. marcescens. Many independent mutations affecting the extracellular enzymes were isolated after chemical and transposon mutagenesis. Using indicator media, we have identified loci involved in the production or excretion of extraceliular protease, nuclease, or chitinase by S. marcescens. None of the mutations represented general extracellular-excretion mutants; in no case was the production or excretion of multiple exoproteins affected. A variety of loci were identified, including regulatory mutations affecting nuclease and chitinase expression. A number of phenotypically different protease mutants arose. Some of them may represent different gene products required for the production and excretion of the major metalioprotease, a process more complex than that for the other S. marcescens exoproteins characterized to date.The movement of molecules from the site of synthesis to a new location is a fundamental property of biological systems. Protein export to the cell envelope has been the object of intensive study in Escherichia coli (4), and mutations affecting this process have been identified (18,30). Many of these mutations are conditional, demonstrating that protein export to the cell envelope is essential to the cell. The extracellular secretion (or excretion) of some proteins into the growth medium can also be achieved by some bacteria (29, 32), although enteric bacteria as a group are not renowned for their ability to excrete proteins. In fact, E. coli only excretes proteins when it carries extrachromosomal elements specifying exoproteins. For some proteins, such as a-hemolysin, bacteriocins, and toxins (14,19,22,26,32), the release of the exoprotein usually requires the presence of other gene products; this implies that E. coli is not normally endowed with a general extracellular secretory system. Since E. coli is such a limited system for studying extracellular proteins, a range of other organisms has been investigated (29,32). Usually the exoproteins these organisms excrete are either toxins or degradatory proteins like proteases and nucleases.Some steps, such as signal sequence (31) recognition and processing, may be common to both envelope and extracellular protein translocation. Genes required for the export of both membrane-bound and extracellular proteins are essential to the cell, but genes required only for excretion appear not to be essential. By isolating mutants that are defective only in the excretion of extracellular proteins, it may be possible to separate this mechanism from envelope secretion.Mutations in Pseudomonas aeruginosa strains defective in the excretion of certain extracellular proteins have been isolated (41) formation of many but not ...