Phytochrome photochromism probed by site-directed mutations and chromophore esterification

The critical amino acid residues that confer photochromism to phytochromes were investigated using N- and C-terminal truncations of phytochrome A and site-directed mutagenesis in the chromophore site. Histidine-324 appears to serve as an anchimeric residue for the photochromism through its H-bonding function. The isoleucine-80 in the chromophore pocket seems to play a catalytic role for chromophore ligation and photochromism. Apparently, the chromophore pocket entails not only the Ile-80 peptide chain but also the amphiphilic alpha-helix backbone around Gln-391.