The measured K_D of the FAP-MG complex is close to the published value of 320nM ^[1].


The measured K_D of the Spinach-DFHBI complex is 537nM ^[2]. Our measured K_D is 100 times smaller than the published K_D. We hypothesized that this could be due to magnesium levels inside bacteria because it has been shown that the binding of DFHBI by Spinach is sensitive to magnesium concentration.

RNA and protein expression levels of T7Lac promoters

The two figures below are plots of representative Spinach and FAP fluorescence over time (from two replicates). The figures compare the fluorescence of three new T7Lac promoters with the wild-type T7Lac promoter, when either 3,5-difluoro-4-hydroxybenzylidene imidazolinone (DFHBI) or Malachite Green (MG) was added. DFHBI is a specific fluorogen that binds to Spinach and MG is a specific fluorogen that binds to FAP. Therefore, we assume that there is a positive correlation between fluorescence values and the amount of either RNA and proteins in bacteria. All fluorescence values are normalized by the corresponding OD600 readings. Please refer to the Time-Lapse protocol in the Protocols page for the full experimental details.

For both the Spinach-DFHBI and FAP-MG plots, fluorescence values increase over time with all promoters. This makes intuitive sense, as we expect the amount of transcribed RNA (reported by Spinach-DFHBI) and translated protein (reported by FAP-MG) to increase with time after inducing cells with IPTG.

The fluorescence level of Mutant I increases more rapidly than the other constructs, indicating that this promoter significantly increases the transcription rate of mRNA from the promoter. Mutant II closely parallels the wild type fluorescence level, being only slightly lower in magnitude. Mutant III's fluorescence level increases very rapidly at first, but seems to be leveling off after one hour. This may indicate that bacteria have adapted host machinery to compensate the metabolic burden.


Mutant I closely parallels the wild-type promoter in terms of magnitudes and expression rates of FAP fluorescence levels. Mutant II exhibits significantly lower fluorescence levels than the wild-type promoter, indicating a slower mRNA translation rates with time. Fluorescence levels of mutant III seems to be increasing at an accelerating rate as compared to the wild-type promoter and reach a significantly higher fluorescence level at the end of the experiment.