Biochemical and Biophysical Characteristics of a Photosynthetic Mutant of Euglena gracilis Blocked in Photosystem II.

A photosynthetic mutant of Euglena gracilis, Z strain, thought to be blocked in the electron transport chain between the two photosystems and to have a missing or nonfunctional primary acceptor for photosystem II, was further studied and characterized. The data from low temperature fluorescence spectra, delayed light emission, and electron paramagnetic resonance support the previous work.The mutant was shown to lack plastoquinone A and possibly cytochrome b(559) but to possess plastoquinone B and a higher complement of carotenoids, especially xanthophylls, than the wild type.The results are consistent with the postulated role of plastoquinone as the primary electron acceptor for photosystem II and as responsible for the electron paramagnetic resonance signal II. The abundance of xanthophylls in the mutant can be explained by the protective role of carotenoids against photosensitized reactions.