Escherichia coli (E. coli) remains the most efficient widely-used host for recombinant protein production. Well-known genetics, high transformation efficiency, cultivation simplicity, rapidity and inexpensiveness are the main factors that contribute to the selection of this host. With the advent of the post-genomic era has come the need to express in this bacterium a growing number of genes originating from different organisms. Unfortunately, many of these genes severely interfere with the survival of E. coli cells. They lead to bacteria death or cause significant defects in bacteria growth that dramatically decrease expression capabilities. In this paper, we review special strategies and genetics tools successfully used to express, in E. coli, highly toxic genes. Suppression of basal expression from leaky inducible promoters, suppression of read-through transcription from cryptic promoters, tight control of plasmids copy numbers and proteins production as inactive (but reversible) forms are among the solutions presented and discussed. Special expression vectors and modified E. coli strains are listed and their effectiveness illustrated with key examples, some of which are related to our study of the highly toxic phage T4 restriction endoribonuclease RegB. We mainly selected those strategies and tools that permit E. coli normal growth until the very moment of highly toxic gene induction. Expression then occurs efficiently before cells die. Because they do not target a particular toxic effect, these strategies and tools can be used to express a wide variety of highly toxic genes.

The regB gene, from the bacteriophage T4, codes for an endoribonuclease that controls the expression of a number of phage early genes. The RegB protein cleaves its mRNA substrates with an almost absolute specificity in the middle of the tertranucleotide GGAG, making it a unique well-defined restriction endoribonuclease. This striking protein has no homology to any known RNase and its catalytic mechanism has never been investigated. Here, we show, using 31P nuclear magnetic resonance (NMR), that RegB produces a cyclic 2',3'-phosphodiester product. In order to determine the residues crucial for its activity, we prepared all the histidine-to- alanine point mutants of RegB. The activity of these mutants was characterized both in vivo and in vitro. In addition, their binding capability was quantified by surface plasmon resonance and their structural integrity was probed by 1H/15N NMR correlation spectroscopy. The results obtained show that only the H48A and the H68A substitutions significantly reduce RegB activity without changing its ability to bind the substrate or affecting its overall structure. Altogether, our results define RegB as a new cyclizing RNase and present His48 and His68 as potent catalytic residues. The effect of the in vivo selected R52L mutation is also described and discussed.

Despite the development of various nonbacterial expression systems, Escherichia coli (E. coli) remains the host of choice for recombinant protein expression. Its culture is simple, fast, inexpensive, and highly efficient (tens of milligrams of pure proteins are typically obtained within 48 hours using as little as 1 liter of culture). Unfortunately, many toxic genes (from various organisms) severely interfere with the physiology of E. coli. As a result, expression yields are dramatically diminished, and sometimes abolished. In fact, some genes are so toxic that E. coli cannot maintain their expression vector during the growth phase (the phase during which recombinant gene expression is presumably repressed). Therefore, modified expression vectors, modified E. coli strains, and appropriate cultivation protocols are needed. This overview discusses several special strategies successfully used to express toxic genes in E. coli.