THE SERRATIA-MARCESCENS NUCE PROTEIN FUNCTIONS AS A HOLIN IN ESCHERICHIA-COLI

The recently discovered nucC locus of Serratia marcescens encodes the cryptic prophage genes nucE, nucD, and nucC. NucC is required for expr ession of the S. marcescens nuclease and functions as a transcriptiona l activator of the nuclease gene, nucA, NucE and NucD are dispensable for nuclease expression but were proposed to allow for secretion of th e nuclease by Escherichia coli, Here, we show (i) that the NucE protei n is membrane bound, (ii) that it can complement the lambda S holin, ( iii) that it can be triggered by potassium cyanide, (iv) that it is de trimental to cell viability, and (v) that the concomitant expression o f nucE and nucD results in cell lysis, Apparently NucE and NucD functi on as a holin and an endolysin, respectively, This suggests that their roles in nuclease secretion by E. coil are indirect, possibly through directed cell lysis.