Novel Biosensors Based on Flavonoid-Responsive Transcriptional Regulators Introduced into Escherichia coli

A remarkable opportunity to expand the existing toolbox for strain engineering is offered by the vast diversity of transcriptional regulators in nature and their exploitation to design metabolite specific sensors. In this study, we describe the construction of two such metabolite biosensors to be applied for the metabolic engineering of Escherichia coli strains. The first metabolite sensor is based on the transcriptional activator FdeR from Herbaspirillum seropedicae SmR1, which activates gene expression of a cyan fluorescent protein in the presence of naringenin. The second metabolite sensor was derived from the transcriptional repressor QdoR from Bacillus subtilis, that when inactivated by quercetin or its biosynthetic precursor kaempferol, results in expression of a green fluorescent protein. Selection of these two transcriptional regulators, an activator and a repressor, displaying opposite regulatory mechanisms demonstrates the general applicability of this enabling technology. Both sensors showed over a 7-fold increase of the fluorescent signal after addition of their specific effectors, and a linear correlation between the fluorescence intensity and flavonoid concentration in the range of 0.01 – 0.1 mM in the medium was observed. The QdoR based sensor was applied for detection of kaempferol production in vivo in E. coli. To this end, the biosensor was introduced in cells containing the flavonol synthase from Arabidopsis thaliana (FLS), which produce kaempferol after addition of dihydrokaempferol to the growth medium. The QdoR based sensor allowed for the monitoring of kaempferol production at the single cell level. Furthermore, the amount of kaempferol produced highly correlated with the specific fluorescence of E. coli cells containing a flavonol synthase gene fls1.These results show that the designed metabolite biosensors can be used to isolate flavonoid producing enzymes in a high throughput manner. This enabling technology is expected to facilitate and shorten the time to develop next generation cell factories for flavonoid production and we are currently working in the field of biosensor development with application in synthetic biology and metabolic engineering.

 

Reference

Siedler, S., Stahlhut, S.G., Malla, S., Maury, J., Neves, A.R., 2014. Novel biosensors based on flavonoid-responsive transcriptional regulators introduced into Escherichia coli. Metab. Eng. 21, 2–8.