Regulatory mutants and transcriptional control of the Serratia marcescens extracellular nuclease gene.

The extracellular nuclease of Serratia marcescens is regulated in a complex fashion. Unlike most catabolic enzymes, it appears not to be substrate regulated. However we have shown it to be regulated by an SOS-like system in S. marcescens. Additionally nuclease expression is regulated in a growth-phase-dependent manner. In this work we demonstrate that growth-phase-dependent regulation is at the transcriptional level. The putative LexA-binding site which mediates SOS regulation is shown to act as an operator site in vivo. The boundaries of a minimal promoter, still regulated by growth phase and SOS regulation, are defined along with the transcriptional start site. However, a region upstream of the nuclease promoter is shown to enhance significantly the expression of nuclease.