Analysis of E. coli phoA ‐ lacZ fusion gene expression inserted into a multicopy plasmid and host cell's chromosome

The production of β‐galactosidase from the E. coli phoA ‐ lacZ fusion gene was studied to compare the gene expression behavior of two cloning methods: insertion to multicopy plasmids and integration into host cell's chromosome. The chromosome‐integrating strain showed more tight control of fusion gene expression levels than the plasmid‐containing strain. A 100‐fold enhancement of specific β‐galactosidase activity in the former strain was achieved in response to changes of initial inorganic phosphate concentration from 1 to 0.1 m M , whereas a 26‐fold increase was observed in the latter strain. The low degree of overexpression in the plasmid‐bearing cells was due to a combination of factors including leaky expression in repressed conditions and limitation of biosynthetic machinery in derepressed conditions. In a mixture of inorganic and organic phosphates, inorganic phosphate levels in the medium exhibited oscillatory behavior. The oscillation of inorganic phosphate is attributed to selective usage of inorganic phosphate followed by hydrolysis of organic phosphate to inorganic by alkaline phosphatase. The fluctuation of inorganic phosphate levels also caused the oscillation of β‐galactosidase activity.