The structure and function of phytochrome A [in plants]: The roles of the entire molecule and of its various parts

Phytochrome A is readily cleavable by proteolytic agents to yield an amino-terminal fragment of 66 kilodalton (kDa), which consists of residues 1 to approximately 600, and a dimer of the carboxy-terminal 55-kDa fragmeng trom residue 600 or so to the carboxyl terminus. The former domain, carrying the tetrapyrrole chromophore, has been studied extensively because of its photoactivity, while less attention has been paid to the non-chromophoric portion until quite recently. However, the evidence gathered to date suggests that this domain is also of great importance. We present here a review of the structure and the biochemical and physiological functions of the two domains, of parts of these domains, and of the cooperation between them. Phytochrome, a blue-green chromoprotein that modulates photomorphogenetic phenomena in plants, exists as two photointerconvertible forms, red-light-absorbing (Pr) and far-red-light-absorbing (Pfr) forms. Of the two forms, Pfr is considered to be the physiologically active form, while Pr is inactive. Thus, an understanding of the differences between the two forms should help us to understand the physiological functions of this molecule. Phytochrome consists of a homodimer of polypeptides of about 120 kDa, each of which carries one open-ring tetrapyrrole known as phytochromobilin. Unfortunately, no crystallographic data have been obtained for either domain, and phytochrome is too large for its structure to be resolved by NMR spectroscopy.