Properties of Phytochrome

Phytochrome is a chromoprotein: this means it is composed of a protein and a chromophore. The chromophore is actually attached to the protein and is responsible for its-colour and light absorption. It is possible to isolate phytochrome in absolutely pure form by following the procedures of protein purification. The phytochrome in test-tubes also shows similar absorption characteristics. It absorbs red light (maximum absorption occurs at 660 nm wavelength) and on conversion, far-red light (maximum absorption occurs in far-red region at 730 nm). A typical absorption spectrum of phytochrome is shown-in figure. It is a soluble protein and is present in the cytoplasm. It is believed that P_fr form may be associated with the membranes. The intact phytochrome protein has a molecular weight of 124,000 daltons. .The total amino acid sequence of the protein is also known and, using recombinant DNA technology, the gene coding for phytochrome has been isolated.

Figure:  Absorption spectrum of phytochrome in vitro. Phytochrome was purified add its Absorption spectrum was measured by a spectrophotometer. When P_r was irradiated with red light, the absorption spectrum (dotted line) was obtained. This shows the conversion of P_r to P_fr.