Abstract
The maintenance of a plasmid vector-host system that selects for bacteria carrying the plasmid without the need for antibiotics is described. In this system, the bacteriophage 434 repressor gene cloned on the plasmid protects the host from lysis by a lambda imm434 cI- prophage. Cells that occasionally lose the plasmid are killed by prophage induction and therefore do not accumulate in the growing culture. The presence of the phage 434 repressor in the cells does not interfere with the process of lambda repressor inactivation and the high-level production of bovine growth hormone.