Overproduction and purification of Mycobacterium tuberculosis WhiB3 in Escherichia coli is enhanced by co-expression...

Members of the WhiB-like (Wbl) family of proteins are found in Acintomycetes and are somewhat recalcitrant to overproduction as soluble proteins in the laboratory protein expression workhorse Esherichia coli. The aim of this study was to evaluate the effects of culture conditions and co-expression of the chaperone protein, trigger factor (TF), on the soluble production of recombinant Mycobacterium tuberculosis (Mtb) WhiB3. A pET28a derived expression plasmid coding for His₆-WhiB3 was created and the effects of varying the concentration of inducer (IPTG), the timing of induction, the nature of the inducer (auto-induction medium) and the temperature of the cultivation on the production of soluble His₆-WhiB3 were tested. Whilst His₆-WhiB3 protein was readily detected, the overwhelming majority of the protein was present in the insoluble fraction of cell-free extracts. However, co-expression of the tig from pTf16, coding for TF, increased His₆-WhiB3 solubility dramatically, facilitating its isolation by affinity chromatography. Purified His₆-WhiB3 was shown to be monomeric, and UV-visible spectra suggested that ∼10% of the isolated protein possessed a [4Fe-4S] cluster. The secondary structural properties of His₆-WhiB3 were altered by acquisition of an iron-sulfur cluster. By developing a protocol to readily overproduce and purify WhiB3, this study paves the way for future structure-function experiments.