We have isolated spontaneousolpmutants ofEscherichia coliK-12 overproducing the periplasmic enzymes β-lactamase (Bla) and alkaline phosphatase (PhoA). Enzyme overproduction was maintained inolpstrains transformed with plasmids carryingbla+andphoA+structural genes, and synthesizing high levels of Bla and PhoA. Transformedolpstrains excreted up to 40 of these enzymes into the growth medium. The introduction of atolAexcretory mutation intoolpstrains led to an increase of enzyme overproduction and a release of 85 of Bla and PhoA enzyme activities into the culture medium
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