Effect of pH on Absorption Spectra of Pea 114 and 121 Kilodalton Phytochromes during and after Red-Light Irradiation

Absorption spectra of pea 114 and 121 kDa phytochromes weremeasured at pH 6.8, 7.8 and 8.8 using a custom-made transientmultichannel spectrum analyzer. The absorption spectra of 114kDa phytochrome as P_R and P_FR were least affected by mediumpH. The absorption spectra at photostationary state under redlight, however, were different under the three different pHconditions, and were different from those obtained 55 s afterred-light irradiation, owing to rapid pH-dependent absorbanceincrease in both red and far-red regions in the dark. In contrast,the absorption spectra of 121 kDa phytochrome were significantlyless affected by medium pH. The absorption spectra measuredat the photostationary state showed a lower P_FR peak at higherpH. The absorption spectra obtained 55 s after the irradiationwere similar under the three pH conditions since the rapid absorbanceincrease in the far-red region in the dark was small. Possibleaccumulation of 114 kDa phytochrome population(s) with low absorbanceat red-light-induced photostationary state at pH 8.8, and theprotective role of the 7 kDa polypeptide at the amino terminusagainst the pH effect in 121 kDa phytochrome are discussed. (Received February 1, 1986; Accepted April 1, 1986)