Determination of protein expression and plasmid copy number from cloned genes in Escherichia coli by flow injection analysis using an enzyme indicator vector

On‐line determination of expression rates from cloned genes in Escherichia coli and of plasmid copy number would be useful for monitoring accumulation of non‐secreted proteins. As an initial model for monitoring gene expression in intact cells, a non‐gene‐fusion enzyme‐based indicator plasmid has been constructed containing the phoA gene coding for alkaline phosphatase (AP) in pUCIS and pACYC184. The activity of AP can be rapidly determined in permeabilized cells. A flow injection analysis (FIA) assay has been developed which allows the direct real‐time measurement of the AP activity during cell growth. A model target gene coding for E. coli cyanase (cynS) has been inserted in order to determine the ratio between the expression of the target and indicator, AP. A linear relationship has been found between plasmid copy number and AP activity for the high‐copy pUC vector. To minimize indicator expression, transcription terminators have been inserted between the cynS and phoA genes, altering the target‐to‐indicator ratio by 10‐ to 40‐fold. These vectors may be useful for the rapid continuous determination of plasmid copy number and target gene expression for nonsecreted proteins and would overcome the limitations of in situ probe biosensors for real‐time determination of the accumulation of proteins from cloned genes in E. coli.