The NucE and NucD lysis proteins are not essential for secretion of the Serratia marcescens extracellular nuclease

The nuclease of Serratia marcescens is an extracellular protein encoded by the nucA gene. Pre-nuclease carries a typical 21-amino-acid N-terminal sign al sequence that interacts with the Sec machinery to allow the translocatio n of nuclease to the periplasm. In Escherichia coli the nuclease remains in the periplasm; however, S. marcescens has the capacity to secrete nuclease extracellularly. The nucC operon carrying the nucEDC genes of S. marcescen s has been identified previously. NucC is a transcriptional activator neces sary for expression of nuclease as well as the extracellular bacteriocin 28 b. NucE resembles and can act as a bacteriophage holin, whereas NucD has ho mology to bacteriophage lysozyme-like proteins. When present on a multicopy plasmid, the nucC operon, and specifically the nucED genes, appeared to al low extracellular secretion of nuclease from E. coli. Here experiments are reported which demonstrate that, when the nucC operon was placed in the E. coli chromosome in single copy, nuclease secretion was lost and nuclease re mained periplasmic. The converse experiment, deletion of the nucE and nucD genes from the chromosome of S. marcescens, likewise had no effect on nucle ase secretion by S. marcescens. It is concluded therefore that NucD and Nuc E are not necessary for nuclease secretion.