RNase E, the major player in mRNA degradation, is down-regulated in Escherichia coli during a transient growth retardation (diauxic lag).

The endoribonuclease RNase E plays a major part in mRNA degradation in Escherichia coli in addition to its role in processing rRNA. RNase E is encoded by an essential gene, rne, also known as ams and hmp, which is autoregulated post-transcriptionally. Here we report a transient decrease in the steady state level of the full-length rne transcript and a corresponding decline in the amount of the protein and enzymatic activity. During this period an mRNA fragment, lacking an intact 5' end, accumulates. This down-regulation of RNase E occurs under aerobic growth conditions in rich medium during a short diauxic lag in mid-exponential phase; it most likely reflects an exhaustion of a not yet identified medium compound which is followed by switching on a new metabolic pathway. During this lag, the levels of bulk protein are maintained. Our results suggest that a transient drop in the intracellular RNase E level is a means of cells to retard mRNA turnover in a period of adjustment to medium utilization. Furthermore, the here described regulation of the rne transcript and its cognate gene product seems to occur by an RNase E-independent mechanism responsive to changes in growth conditions.

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 expression of the S. marcescens nuclease and functions as a transcriptional activator of the nuclease gene, nucA. NucE and NucD are dispensable for nuclease expression but were proposed to allow for secretion of the nuclease by Escherichia coli. Here, we show (i) that the NucE protein 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 detrimental to cell viability, and (v) that the concomitant expression of nucE and nucD results in cell lysis. Apparently NucE and NucD function as a holin and an endolysin, respectively. This suggests that their roles in nuclease secretion by E. coli are indirect, possibly through directed cell lysis.