Downregulation of T7 RNA polymerase transcription enhances pET-based
recombinant protein production via suppression of Escherichia coli BL21
(DE3) programmed cell death
Abstract
E. coli BL21 (DE3) is an excellent and widely used host for recombinant
protein production. Many variant hosts were developed from from BL21
(DE3), but improving the expression of specific proteins remains a major
challenge in biotechnology. In this study, we found that when BL21 (DE3)
overexpressed glucose dehydrogenase (GDH), a significant industrial
enzyme, serious autolysis was induced. Subsequently, we observed this
phenomenon in the expression of 10 other recombinant proteins. This
precludes a further increase of the produced enzyme activity by
extending the fermentation time, which is not conducive to the reduction
of industrial enzyme production costs. The membrane structure and mRNA
expression analysis showed that cells suffered programmed cell death
(PCD) during autolysis period. However, blocking three known PCD pathway
in BL21 (DE3) cannot alleviate autolysis completely. Furthermore, we
attempted to develop a strong expression host resistant to autolysis by
controlling the speed of recombinant protein expression. To find a more
suitable protein expression rate, the high- and low-strength promoter
lacUV5 and lac were shuffled and recombined to yield the promoter
variants lacUV5-1A and lac-1G. The results showed that only one base in
lac promoter needs to be changed, and the A at the +1 position was
changed to a G, resulting in a host of BL21 (DE3-lac1G), which
successfully withstand the PCD of the host. The GDH activity at 43h was
greatly increased from 37.5 U/mL to 452.0 U/mL. In scale-up
fermentation, the new host was able to produce the model enzyme with a
high rate of 89.55 U/mL/h at 43h, compared to the 3 U/mL/h of BL21
(DE3). Importantly, BL21 (DE3-lac1G) also successfully improved the
production of other 10 enzymes. The engineered E. coli strain in the
study conveniently optimizes recombinant protein overexpression by
suppressing cell autolysis, and shows potential industrial applications.