Engineered hybrid MerR transcription factors enable construction of novel MerR based synthetic circuits for heavy metal detection in Bacillus subtilis

Anthropogenic activity including burning of fossil fuels, overuse of pesticides and fertilisers and the release of contaminated industrial effluent has substantially increased toxic heavy metal deposition into the environment. Conventional analytical methods for monitoring heavy metal pollution are hampered by large equipment costs, the requirement for trained personnel, and inability to quantify heavy metal bioavailability. Bacterial biosensors offer a promising alternative to these methods. The well-characterised genetics of Bacillus subtilis and its ability to produce spores for long-term storage make it an ideal choice for use in biosensor development. Although B. subtilis possesses innate metal detection systems, different mechanisms of transcriptional regulation, variation in metal specificity and sensitivity impedes flexible synthetic biosensor circuit design for metal detection. By contrast, metal-sensitive MerR family regulators possess a conserved and tightly controlled mechanism of transcriptional regulation with both high sensitivity of detection and metal specificity. However, as the majority of diversity in metal specificity in MerR is found in Gram-negative species, species-specific differences in biological part usage limits their implementation in Gram-positive species, such as B. subtilis. To overcome this design constraint, a Hg2+-specific MerR system from Staphylococcus aureus was modified to produce a novel Zn2+-sensing hybrid MerR regulator by fusion of the MerR DNA-binding domain with the metal-binding domain from the Zn2+-responsive MerR homologue ZntR. This yielded a functional hybrid (MerRZntR) with preferential detection for Zn2+ (Zn2+>Hg2+). This work demonstrates a potentially flexible approach to design modular metal detection circuits using additional MerR hybrids based on Gram-negative systems for use in B. subtilis biosensor development.