A glucose-insensitive T7 expression system for fully-induced expression of proteins at a subsaturating level of L-arabinose.

The L-arabinose (Ara)-controlled T7 expression system was previously constructed by creation of an Escherichia coli BL21(BAD) strain. The production of recombinant proteins in this strain was stringently regulated and reached a high level upon induction with Ara. Nevertheless, this system is still associated with inherent problems of interference with glucose and of the all-or-nothing induction profile at a subsaturating level of Ara. In this study, these problems were circumvented by modifying the physiological traits of BL21(BAD) strain. This was followed by deletion of ptsG gene and the araFGH and araBAD operon. The former encodes the glucose transporter while the latter two gene operons produce proteins responsible for Ara uptake and catabolism. In addition, the expression of genomic araE (encodes the Ara transporter) was constitutively enhanced. The resulting strain was designated BAD-5. By expression of the faster degrader GFP(LAA) at a subsaturating level of Ara, 80% of BAD-5 strain was found visually bright in the presence or absence of glucose. A further analysis by flow cytometry showed a uniform distribution of GFP expression for BAD-5 strain. In marked contrast, BL21(BAD) strain exhibiting visual brightness was less than 10% of the cell population and remained dark in the presence of glucose. Moreover, a saturated level of luciferase from Renilla reniformis (Rluc) could be readily obtained in BAD-5 strain at 20 μM Ara regardless of glucose. Rluc in BL21(BAD) strain was produced in an Ara dose-dependent manner, and the protein production became arrested when glucose was present. Overall, it illustrates the usefulness of the improved system for overproduction of recombinant proteins in an efficient, homogeneous, and glucose-insensitive way.