life11e_ch36

Phytochrome stimulates gene transcription

How does phytochrome, or more specifically P_fr, work? Phytochrome has two subunits (Figure 36.14), each composed of a protein chain and a chromophore. Gene transcription is stimulated when P_r is converted to the P_fr isoform. When P_r absorbs red light, the chromophore changes shape, causing change in the conformation of the protein itself, from the P_r form to the P_fr form. Conversion to the P_fr form exposes two important regions of the phytochrome protein, both of which affect transcriptional activity:

Exposure of a nuclear localization sequence (see Figure 14.18) results in movement of P_fr from the cytoplasm to the nucleus. Once in the nucleus, P_fr binds to transcription factors and thereby stimulates expression of genes involved in photomorphogenesis.
Exposure of a protein kinase domain causes P_fr protein to phosphorylate itself and other proteins involved in red-light signal transduction, resulting in changes in the activity of transcription factors.

Figure 36.14 Phytochrome Stimulates Gene Transcription Phytochrome is composed of two polypeptide chains, each with a chromophore. This pair of polypeptides undergoes a conformational change upon absorbing light. When phytochrome absorbs red light, it converts to the P_fr form, which activates transcription of phytochrome-responsive genes.

The effect of activating these transcription factors is quite large: in Arabidopsis, phytochrome affects an amazing 2,500 genes (10 percent of the entire genome!) by either increasing or decreasing their expression. Some of these genes are related to hormones. For example, when P_fr is formed in seed germination, genes for gibberellin synthesis are activated and genes for gibberellin breakdown are repressed. As a result, gibberellins accumulate and seed reserves are mobilized.