Abstract

Plasmids are accessory genetic elements in bacteria which carry non-essential but often very important genes such as those determining pathogenicity or antibiotic resistance. Being physically independent of the bacterial chromosome plasmids have evolved sophisticated mechanisms to ensure their faithful transmission to daughter cells at division. Studies of these mechanisms in E. coli led to the discovery of a cell cycle checkpoint which prevents division of the host bacterium if survival of the plasmid is at risk. By exploiting our understanding of the checkpoint mechanism it has been possible to re-engineer the bacterial cell so that it becomes an efficient factory for the production of proteins or metabolites. These quiescent E. coli cells (Q-Cells) are non-growing but remain metabolically active, and this allows cellular resources to be focussed on the production of recombinant proteins rather than unwanted biomass. Initial attempts to commercialise this system were hampered by its relative complexity but recent advances in our understanding of the underlying mechanism have led to the development of a second generation quiescent cell factory which is both cheap and user-friendly.