High production of heterologous proteins in Escherichia coli using the thermo-regulated T7 expression system

The exclusive use of isopropyl beta-D-thiogalactopyranoside to activate the T7 promoter for protein production has limited the general use of the expression system. We have sought an alternative by constructing a recombinant Escherichia coli strain, BL21 (G2), to carry a chromosomal copy of T7 gene 1 fused to the lambdaP(L) and,PR tandem promoter. As a result, the recombinant strain harboring the carbamoylase gene from Agrobacterium radiobacter NRRL B11291 was shown to display various levels of protein production in response to different degrees of heat shock. In particular, the system remained inactive at 30degreesC and exhibited high sensitivity to heat such that a detectable carbamoylase activity could be measured after exposure to 33degreesC. Moreover, heating in two steps - elevating the temperature from 30degreesC to 39degreesC and holding for a brief period, followed by reducing to 37degreesC - was found to be the most potent method for protein production in this case. Using this approach, the recombinant protein accounted for 20% of total protein content of the cell. These results reveal the advantages of this expression system: responsiveness to thermal modulation and high-level production capability. In an attempt to enhance the total protein yield, a fed-batch fermentation process was carried out to control the cell growth rate by adjusting the substrate inflow. By applying the two-step temperature change, a carbamoylase yield with enzyme activity corresponding to 14,256 units was obtained. This production yield is a 10-fold increase in comparison with that at the batch-fermentation scale and 2,000-fold higher than that achieved at the shake-flask scale. Overall, it illustrates the promise of the newly constructed T7 system based on heat inducibility for industrial scale production of recombinant proteins.