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Carbonylcyanide m -chlorophenylhydrazone is a powerful inhibitor of both photosynthetic phosphorylation and photoreduction of O 2 in isolated spinach chloroplasts. The inhibition of phosphorylation was observed in three cyclic systems (PMS, vitamin K 3 and FMN) and in three non-cyclic systems (ferricyanide-water, TPN + -water and TPN + -ascorbate). Inhibition of O 2 evolution was also obtained in the ferricyanide-water and TPN + -water systems but only at higher concentrations of CCCP. The only photochemical activity of chloroplasts which was found to be resistant to CCCP inhibition was the photoreduction of TPN + with ascorbate-dichlorophenol-indophenol as the electron donor (TPN + -ascorbate system). In the cyclic system (PMS) but not in the non-cyclic system (ferricyanide-water) CCCP inhibition of photophosphorylation varied with light intensity. Thus, in the PMS system, at 1000 lux, 0.4 μM CCCP gave 50% inhibition of photophosphorylation, whereas at 50 000 lux, 8 μM CCCP was required to give 50% inhibition. A model is proposed to account for the differences between cyclic and non-cyclic phosphorylation. The inhibition of photophosphorylation by CCCP was completely reversed by the addition of cysteine or by washing.

1. 1. The optical rotatory dispersion (ORD) and circular dichroism (CD) of phytochrome in the PR and PFR forms were determined from 200 to 500 nm. Both the ORD and the CD spectra showed dramatic differences between the PR and PFR in this wavelength range. 2. 2. The CD spectra below 240 nm correspond closely with that assigned to the β-chain configuration. If, however, the protein is considered to be a mixture of α-helix and random coil, the ellipticity at 220 nm would correspond to an α-helix content of 14%.