Reactor takeover by plasmidless cells is i major problem encountered when produc ing proteins from plasmid-borne genes ii genetically engineered bacteria. We have approached this problem by deleting the essential ssb gene from the Escherichia col chromosome and placing it on a plasmid Plasmidless cells do not accumulate ever after growing such strains under non-se lective continuous culture conditions foi extended periods of time. Other ssb-con taining plasmids can be readily intro duced into this E. coli strain by a plasmid displacement technique. Using this sys tem, we have achieved very high levels o: 3-lactamase production in continuous cul ture without selective pressure.
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