In cytokine receptors, 

the receptor and kinase are separate polypeptides, encoded by different genes, yet are bound tightly together.

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Phytochromes are photoreceptors that absorb red and far-red light most strongly (600–750 nm)

they also absorb blue light (350-500 nm) and UV-A radiation (320–400 nm).

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The amount of light is referred to as the fluence, which is defined as the total number of photons impinging on a unit surface area. Total fluence = fluence rate × the length of time

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Bacterial phytochrome-like proteins (BphPs) regulate the biosynthesis of the photosynthetic apparatus in Rhodopseudomonas

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In dark-grown or etiolated plants, phytochrome is present in a red light–absorbing form, referred to as Pr. Pr, which to the human eye is blue, is converted by red light to a far-red light–absorbing form called Pfr, which is blue-green. 

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Pfr, in turn, can be converted back to Pr by far-red light known as photoreversibility

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When Pr molecules are exposed to red light, most of them absorb it and are converted to Pfr, but some of the Pfr also absorbs the red light and is converted back to Pr because both Pr and Pfr absorb red light.