Method of conditionally controlling the survivability of a recombinant
cell population and of containing such cells to an environment or
containing replicons to a host cell is based on the use of proteic killer
systems including the E. coli relBE locus and similar systems found in
Gram-negative and Gram-positive bacteria and Archae. Such system are
generally based on a cytotoxin polypeptide and an antitoxin or antidote
polypeptide that in contrast to the cytotoxin is degradable by proteases.
The recombinant cells are useful as vaccines, pollulant degrading
organisms or as biogical pest control organisms e.g. expressing B.
thuringiensis crystalline proteins.