Photosynthetic Electron Transport Chain of Chlamydomonas reinhardi. III. Light-Induced Absorbance Changes in Chloroplast Fragments of the Wild Type and Mutant Strains

Abstract

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).

Energy and Electron Transfer Systems of Chlamydomonas reinhardi. I. Photosynthetic and Respiratory Cytochrome Systems of the Pale Green Mutant

“…is redutced by ascorbate, and supposed to be located in a non-cy-clic pathway in the place Levine and Gormani (16) have shown that a spectrumn of light-induced oxidation of chloroplast fragments from Chlam"Ydomionas reinhardi has a max,inmum at 560 nm wlhich might be attributed to the cy.tochrome b559 found bv others. However, they failed to see light-induced oxidation of b563 in their preparation, in which this component had probably been completely oxidized during sonication (cf.…”

“…The presence of 2 types of cvtochrome b, cvtochrome b563 or be, and cvtochrome b559, in photosyntlhetic ellectron transfer svstemls has been pointed otit in spinach chloroplasts (2,5,1) and in Chlamnydomtolnas reinhardi (15,16,24). Cytochrome b563 may be auto-oxidizable (13), is not reduced by ascorbate, and has recentlv been considered to be a member of a cyclic pathway around photosystem I in spinach chloroplasts (5, l) atnd Chlainydonmonas (9,15).…”

Detecting Mutants That Have Impaired Photosynthesis by Their Increased Level of Fluorescence

“…The unicellular green algae Chlarnydowiionas recinliardi and Scenedesmnius obliqutus have been used in this way (1)(2)(3)6 For NADP-a the reaction mixture (2.0 ml) contained chloroplast fragments (20-25 ug chlorophyll) and the following in umoles: potassium phosphate buffer, pH 7.0, 20; KCI, 40; MgCl2, 5; ferredoxin purified from wildtype C. rcinhardi, 0.0005; and NADP, 0.5. For NADP-b the reaction mixture contained, in addition to the above, the followitng in Anmoles: DPIP, 0.1; sodium ascorbate, 10; and DCM1IU, 0.02.…”

“…A technique is described for detecting mutants of Chlaryvdoiimoias rciniar'di that lhave impaired photosynthesis. It is based on the fact that when photosynthetic electron transport is blocked there may be an increase in the level of fluorescence similar to that obtained in the presence of an inhibitor of photosynthetic electron transport such as dichloropohenyldimethylurea.One way to obtain information about the mechanism of photosynthesis is to study mutant organisms that are unable to carry out normal photosynthesis.The unicellular green algae Chlarnydowiionas recinliardi and Scenedesmnius obliqutus have been used in this way (1)(2)(3)6 …”

“…The unicellular green algae Chlarnydowiionas recinliardi and Scenedesmnius obliqutus have been used in this way (1)(2)(3)6 …”