abstract
This study describes the construction of two flavonoid biosensors, which can be applied for metabolic
engineering of Escherichia coli strains. The biosensors are based on transcriptional regulators combined
with autofluorescent proteins. The transcriptional activator FdeR from Herbaspirillum seropedicae SmR1
responds to naringenin, while the repressor QdoR from Bacillus subtilis is inactivated by quercetin and
kaempferol. Both biosensors 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 externally added
flavonoid concentration. The QdoR-biosensor was successfully applied for detection of kaempferol
production in vivo at the single cell level by fluorescence-activated cell sorting. Furthermore, the amount
of kaempferol produced highly correlated with the specific fluorescence of E. coli cells containing a
flavonol synthase from Arabidopsis thaliana (fls1). We expect the designed biosensors to be applied for
isolation of genes involved in flavonoid biosynthetic pathways.