Fig. 2. Overview of sensing mechanisms used in dynamic metabolic control.

(A) A chemical inducer binds to a repressor that undergoes a conformational change and releases itself from binding to DNA, thus allowing RNA polymerase to promote gene expression (Soma et al., 2014). (B) As cell density increases, more AHL molecules are released into the extracellular environment until the concentration is high enough to trigger a quorum response (Gupta et al., 2017). (C) Thermolabile repressor cI857 changes conformation and DNA binding activity in response to temperature (Zhou et al., 2012). (D) Light responsive transcription factors are created by fusing light-sensitive protein domains with DNA binding domains (Milias-Argeitis et al., 2016). (E) The P_gas promoter is able to detect and respond to low pH levels (Yin et al., 2017). (F) FadR is a repressor transcription factor that binds to acyl-CoA thus acting as a sensor for acyl-CoA (Zhang et al., 2012). (G) The L-Trp biosensor is based on a metabolite-responsive signal peptide TnaC (Fang et al., 2016). Without L-Trp, the operon undergoes the Rho-dependent transcriptional termination. L-Trp can prevent the release of the ribosome at the tnaC stop codon, blocking Rho from binding to the mRNA, so that RNA polymerase can continue to transcribe the downstream GOI.