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In this study, a colorimetric whole-cell biosensor for cadmium (Cd) was designed using a genetically engineered red pigment producing bacterium, Deinococcus radiodurans. Based on the previous microarray data, putative promoter regions of highly Cd-inducible genes (DR_0070, DR_0659, DR_0745, and DR_2626) were screened and used for construction of lacZ reporter gene cassettes. The resultant reporter cassettes were introduced into D. radiodurans R1 to evaluate promoter activity and specificity. Among the promoters, the one derived from DR_0659 showed the highest specificity, sensitivity, and activity in response to Cd. The Cd-inducible activity was retained in the 393-bp deletion fragment (P0659-1) of the P0569 promoter, but the expression pattern of the putative promoter fragments inferred its complex regulation. The detection range was from 10 to 1 mM of Cd. The LacZ expression was increased up to 100 μM of Cd, but sharply decreased at higher concentrations. For macroscopic detection, the sensor plasmid (pRADI-P0659-1) containing crtI as a reporter gene under the control of P0659-1 was introduced into a crtI-deleted mutant strain of D. radiodurans (KDH018). The color of this sensor strain (KDH081) changed from light yellow to red by the addition of Cd and had no significant response to other metals. Color change by the red pigment synthesis could be clearly recognized in a day with the naked eye and the detection range was from 50 nM to 1 mM of Cd. These results indicate that genetically engineered D. radiodurans (KDH081) can be used to monitor the presence of Cd macroscopically.

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