Overexpression of streptokinase using a fed-batch strategy

The structural gene for streptokinase was cloned under the T7 promoter containing the ompA secretory signal sequence. The T7 RNA polymerase was induced with IPTG when the biomass concentration was 1.2 mg dry cells/ml and pulse feeding of concentrated substrate was done at different time intervals. The biomass concentration reached 1.8 mg dry cells/ml and the activity obtained in the supernatant was 180 plasmin units/ml compared to 17 plasmin units/ml obtained by growing and inducing the cells in simple LB medium. The results indicate that high expression levels can be obtained with high cell density cultivation in the T7 system by using a proper feeding strategy.