IGEM:UNAM/2009/Notebook/Modeling logbook Claudia/2010/06/08

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Objective: Calculate the photon flux that activates the red light photoreceptor Cph8.

The chimaeric Cph8 photoreceptor, composed by the cyanobacterial phytochrome (Cph1) fused to the EnvZ histidine kinase domain from E.coli, under dark conditions functions as a phosphotransfer from EnvZ to OmpR, which in a phosphorylated state activates the transcription from the promoter ompC. But, in the presence f red light the activity of the EnvZ histidine kinase domain is inhibited, preventing the phosphorylation of OmpR. Thus, the Cph8 photoreceptor is repressed as a function of red light.

In Tabor J et al., the production of b-galactosidase under the ompC promoter was used as a system to report the output response of the photoreceptor Cph8 to different light intensities at 650 nm. The maximum repression was achieved at ~ 0.04W/m2 to 0.10 W/m2, meaning that at this range of light intensity the Cph8 photoreceptor is fully repressed or inactivated.

Using the following formula:

```The energy E of a photon is :
E = hc / λ
```

where h is Planck's Constant; h = 6.626176 x 10^-34 J•s

The velocity of any photon is c, the speed of light. 3 * 10^8 m/s

And λ, the wavelength of the photon that in this case is 650 x 10^-9 m ( 650 nm).

Thus,

```E=  ((6.63 * (10^(-34))) * (3 * (10^8))) / (650 * (10^(-9)))
E= 3.06 × 10^-19 J
```
```The photon flux density (Φ ) at a given light intensity is:
Φ= P/ E
```

where P is the light intensity in Watts and E the energy of a photon in Jules.

Substituting:

```P= 0.04 W and E= 3.06 × 10^-19 J.
Φ= 0.04/ (3.06 * 10^-19 ) = 1.30718954 × 10^17 photons/s*m2
```