Engineering a Rhamnose-Inducible System to Enhance the Extracellular Electron Transfer Ability of Shewanella Genus for Improved Cr(VI) Reduction

Members of the Shewanella genus play important roles in bioenergy generation and environmental bioremediation, but their practical applications in engineered systems are still limited due to the lack of effective synthetic biology tools and parts such as promoters and expression systems. In this work, the rhaBAD promoter of Escherichia coli was introduced into various Shewanella species including S. oneidensis MR-1, S. decolorationis S12, and S. aquimarina SW-120 to enhance their extracellular pollutant transformation ability with Cr(VI) reduction as an example. The rhaBAD promoter had low leaky expression without L-rhamnose induction and exhibited a linear response to a wide concentration range. The inducer was nontoxic and could not be metabolizable by the Shewanella species. In addition, a unimodal induction of the rhaBAD promoter was found at the single-cell level. Applying this rhamnose-inducible system to the Shewanella species allowed for precise control on their extracellular electron transfer abilities to promote Cr(VI) transformation. The engineered strain induced by L-rhamnose reduced 98% Cr(VI) within 9 h, while only 59.1% Cr(VI) reduction was achieved by the control strain. Moreover, a 3-fold higher Cr(VI) reduction rate for the engineered strain induced by L-rhamnose confirms the improvement of its biotransformation ability. Our work provides an efficient regulation tool for engineering Shewanella species and may hopefully bring them a step further toward practical applications.