Production of Soluble Human Interleukin‐6 in Cytoplasm by Fed‐Batch Culture of Recombinant E. coli

The major objective of this study is to identify fed‐batch culture conditions optimal for the production of human interleukin‐6 (hIL‐6) in a soluble form. Five different expression vectors were constructed for the expression of hIL‐6 and hIL‐6s fused with NusA, maltose binding protein (MBP), thioredoxin (Trx) or ubiquitin (Ubi). A series of flask cultures were conducted in LB medium at 37 °C. The intact hIL‐6 was expressed mostly in the form of inclusion body. More than 95% of the hIL‐6 fused with NusA (NusA/hIL‐6) and about 90% of MBP/hIL‐6 were expressed in a soluble form, whereas Trx/hIL‐6 and Ubi/hIL‐6 were expressed mostly in the form of inclusion body. Based on this result, NusA was selected as the fusion partner for the production of hIL‐6 in the subsequent experiments. A series of pH‐stat fed‐batch cultures of an E. coli BL21(DE3) transformed with a NusA/hIL‐6 expression vector were conducted in a bioreactor with a working volume of about 3 L. As the amount of nitrogen source was increased in the feeding medium, more soluble NusA/hIL‐6 was produced, while the total amount was not significantly changed. Under the best conditions tested, about 90% of NusA/hIL‐6 was produced in the soluble form. In this case, the concentration of soluble NusA/hIL‐6 was 7.5 g/L with a volumetric productivity of 0.43 g/L‐h.