SOME PROPERTIES OF PHOTOTRANSFORMATION OF RYE PHYTOCHROME IN VITRO

Abstract— Much of the experimental data in the phytochrome literature has been obtained using a small‐molecular‐weight protein fragment. Hence, several properties of phototransformation were re‐examined using large‐molecular‐weight rye phytochrome. The kinetics of phototransformation are first‐order, both for the conversion of Pr to Pfr and for the reverse reaction. The quantum yield of phototransformation was found to be 0·28 mol Einstein^‐1 for the conversion of Pr to Pfr and 0·20 mol Einstein^‐1 for the conversion of Pfr to Pr. Intermediates in phototransformation were measured by cycling the pigment with high‐intensity mixed red and far–red light. The difference spectrum of these intermediates between 367 and 575 nm was found to be similar to that previously reported for oat and pea phytochrome. Analysis of intermediate decay indicated complex kinetics and not a single first‐order species. Transient absorbancy changes in the blue region of the spectrum upon actinic illumination could be attributed to differential rates of initial bleaching of the two forms of the pigment and a consequent alteration in the proportion of the two forms in the mixture until photostationary equilibrium is re‐established.