Question

Phytochrome occurs in two forms. It's stable form is

• A. P${}_{fr}$ form
• B. P${}_r$ form
• C. Both of (A) and (B)
• D. None of the above

Answer 1

The correct option is C Both of (A) and (B)

The red light sensing photoreceptors, the phytochromes, are a single family of proteins with amino acid sequence homology spanning from cyanobacteria to non-photosynthetic bacteria to plants. Phytochromes are synthesized in the inactive form, for which the absorption maximum is 660 nm. The red absorbing form, Pr, on absorption of a photon, converts to a form with an absorption maximum of 730 nm, Pfr (far red). The Pfr form is the active form, but it can be converted back to Pr (and inactivated) by the absorption of a far-red photon.
The interconvertability of the two forms of phytochrome is an important feature for sensing light intensity as well. In any natural light environment there will be photons of wavelengths that can be absorbed by each of the two forms of phytochrome. This results in individual molecules of phytochrome cycling between the two forms.

Thus, the correct answer is option C.