The Serratia marcescens extracellular nuclease is a secreted protein that is subject to growth phase and SOS control. Regulatory mutants affecting nuclease expression have been isolated that define a new locus, nucC, essential for transcription of the nuclease gene nucA. The cloned nucC gene is able to activate efficient expression from the nucA promoter in Escherichia coli, where it normally is poorly expressed. NucC is very similar to the bacteriophage P2 Ogr protein, a transcriptional activator essential for P2 late gene expression. NucC is able to replace P2 Ogr to support the growth of P2 ogr- mutants in E. coli. Ogr is a poor activator of the nuclease promoter in E. coli, but the related delta gene product from satellite phage P4 is highly effective. The presence of genes encoding a lysozyme and a putative porin or holin in the nucC operon suggests that nucC may be part of a cryptic prophage genome. The putative holin-like membrane protein is required in E. coli for extracellular secretion of the S. marcescens nuclease.