Biosensors and their applications in microbial metabolic engineering Fuzhong Zhang, Jay Keasling  Trends in Microbiology  Volume 19, Issue 7, Pages 323-329 (July 2011) DOI: 10.1016/j.tim.2011.05.003 Copyright © 2011 Terms and Conditions

Figure 1 Examples of engineered cellular biosensors. (a) Engineering of a two-component system as an environmental sensor. A light-sensing domain (LOV from the B. subtilis protein YtvA) was genetically fused to the HK domain of FixL from the FixL-FixJ two-component system of B. japonicum [22]. Blue light irradiation represses the kinase activity of FixL on FixJ, which in turn represses the target gene expression. The red circle represents the phosphoryl group. (b) The E. coli chemotaxis signaling pathway was engineered to create a cellular biosensor for extracellular aspartate [42]. The intracellular domain of the aspartate-sensing protein Tar was replaced by EnvZ from the E. coli EnvZ-OmpR two-component system. The engineered cells were able to express target genes corresponding to extracellular aspartate concentration. Green circles represent aspartate molecules. (c) A RNA-based biosensor for the intracellular chemical theophylline [52]. The engineered riboswitch binds to theophylline, exposing a single-strand RNA sequence, which is complementary to a target mRNA and regulate translation of the mRNA. Green circles represent theophylline molecules. Trends in Microbiology 2011 19, 323-329DOI: (10.1016/j.tim.2011.05.003) Copyright © 2011 Terms and Conditions

Figure 2 Cellular biosensor design based on a metabolite-responsive transcriptional factor. The engineered pathway is illustrated. MetA and MetB represent metabolic intermediates. Promoters PA and PB control the expression of gene cluster GeneA–GeneB and GeneC–GeneD, respectively. MTF is a metabolite-responsive transcriptional factor, whose DNA-binding activity is antagonized by MetB. Promoter PA can be engineered to be activated by both MTF and the carbon source using an AND logic gate. Promoter PB can be engineered to be repressed by MTF and activated by another signal if necessary. When biosynthesis starts, no MetB has been synthesized. MTF represses PB promoter and the downstream pathway is turned off to prevent unnecessary protein expression. PA is activated by both MTF and the presence of the carbon source. The expressed genes convert MetA to MetB. When the MetB concentration accumulates to a sufficient level, it binds to MTF and inhibits the DNA-binding activity of MTF. PB is turned on, and the resulting proteins convert MetB to the final product. Meanwhile, when MetB concentration is too high, it inhibits the activation of PA by MTF, and MetB biosynthesis is turned down to prevent MetB overaccumulation. Trends in Microbiology 2011 19, 323-329DOI: (10.1016/j.tim.2011.05.003) Copyright © 2011 Terms and Conditions