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Sum;inary. Light-induced absorbance changes were investigated in chloroplast fragments of wild type Chlamiiydomitonas reinhardi and 5 different mutant strains having impaired photosynthesis. Two absorbance changes were detected, 1 hftving a maximtum at 553 nm and the other at 559 nm. The component exhibiting the 553 nm change is a cytochrome similar to cytochrome f from higher plant chloroplasts. The component exhibiting the 559 nm change has the properties of a cytochrome similar to cytochrome b,. Two of the mutant strains (aic-115 and ac-141) were foutnd to lack the 559 cytochrome and light induced only the oxidation of the 553 cytochrome. A third mutant strain (ac-206), previously shown to lack the 553 cytochrome, exhibited only the lightindtuced reduction of the 559 cytochrome. A fourth mutant strain (ac-208), shown to lack plastocyanin, exhibited absorbance changes attributable to both cytochromes. However, light was capable of inducing the reduiction of the 559 cytochrome btut not its oxidation. On the other hand, light induiced the oxidation of the 553 cytochrome btut not its reduction.These observations are disctussed in terms of the series formulation for photosynthetic electron transport in which the 559 cytochrome is reduced by system II and transfers electrons via the component affected in aic-21 to the 553 cytochrome. Accordingly, system I sensitizes the oxidation of the 3 components of the electron transport chain.Reactions of the photosynthetic electron transport chain of the.unicellular green alga Chlantydomionas rcinzardi have been investigated using the wild type strain and mutant strains that are incapable of carrying ouit normal photosynthesis (4,8,9,12). The pturpose of the investigations with the mutant strains is to determine the nature of the componenits of the photosynthetic electron transport chain and the sequience in which they act (8). For example, recent investigations of NADP photoreduiction and photosynthetic phosphorylation with 2 different mutant strains (4), one lacking a c-type cytochrome, the other plastocyanin, stuggested that these 2 chloroplast components function in series in the photosynthetic electron transport chain and that their sequtence is system II -) cvtochrome 553 e plastocyaniin -system I, where system I anl system II refer to 2 different photochemical reaction centers (3).

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