Photosynthetic phosphorylation in Chlamydomonas reinhardi. Effects of a mutation altering and ATP-synthesizing enzyme

Reminiscences of Robert Paul Levine (1927-2022)

I present my personal reminiscence of Paul Levine-a highly innovative scientist who did seminal work in photosynthesis. He was among the first to initiate and use a genetic approach toward photosynthesis. He greatly helped in establishing the green unicellular alga Chlamydomonas reinhardtii as a powerful model system not only for understanding the function of the photosynthetic apparatus but also for studying its biogenesis and regulation. During the period he spent at Harvard, he made several ground-breaking contributions such as identifying and establishing the order of some components of the photosynthetic electron transport chain as well as determining their genetic origin. He trained many students and post-doctoral fellows several of whom later became prominent in this field and in other areas of plant science.

Historical perspective on Chlamydomonas as a model for basic research: 1950-1970

During the period 1950-1970, groundbreaking research on the genetic mapping of Chlamydomonas reinhardtii and the use of mutant strains to analyze photosynthesis was conducted in the laboratory of R. Paul Levine at Harvard University. An account of this era, based in part on interviews with Levine, is presented.

Phylogenomic analysis of the Chlamydomonas genome unmasks proteins potentially involved in photosynthetic function and regulation

Chlamydomonas reinhardtii, a unicellular green alga, has been exploited as a reference organism for identifying proteins and activities associated with the photosynthetic apparatus and the functioning of chloroplasts. Recently, the full genome sequence of Chlamydomonas was generated and a set of gene models, representing all genes on the genome, was developed. Using these gene models, and gene models developed for the genomes of other organisms, a phylogenomic, comparative analysis was performed to identify proteins encoded on the Chlamydomonas genome which were likely involved in chloroplast functions (or specifically associated with the green algal lineage); this set of proteins has been designated the GreenCut. Further analyses of those GreenCut proteins with uncharacterized functions and the generation of mutant strains aberrant for these proteins are beginning to unmask new layers of functionality/regulation that are integrated into the workings of the photosynthetic apparatus.

A nuclear mutant of Chlamydomonas reinhardtii defective in photosynthetic photophosphorylation. Characterization of the algal coupling factor ATPase

Use of the Flagyl selection procedure [Schmidt et al. (1977) Proc. Natl Acad. Sci. USA, 74, 610-614] led to the isolation of a nuclear mutant of Chlamydomonas reinhardtii designated thm-24. This mutant displays normal electron transport rates in vitro, possesses high latent ATPase activity bound to the thylakoid membrane, but is incapable of photophosphorylation. Decay of the transmembrane potential, as indicated by the kinetics of the 520-nm absorption change after illumination, is unusually slow and markedly biphasic. Sodium dodecylsulfate/polyacrylamide gel electrophoresis of purified thylakoid membranes shows mutant thm-24 to be lacking a number of polypeptides including those previously designated 4.1, 4.2 and 8.1. Treatment of purified thylakoid membranes of wild-type and mutant algae, using the chloroform-release procedure of Beechey et al. [(1975) Biochem. J. 148, 533-537] resulted in the removal of ATPase activity from each strain. In wild-type cells, the ATPase activity was of heterogeneous enzymatic origin; fractionation of the chloroform-release extracts by non-denaturing polyacrylamide gel electrophoresis yielded three distinct bands displaying ATPase activity, designated ATPases I, II and III. In contrast, extracts from membranes of mutant thm-24 yielded only one ATPase-containing fraction, co-migrating with ATPase I from wild-type. Use of electrophoretic, immunological and enzymatic methods established a correspondence of the polypeptide subunits of ATPases II and III and those of spinach coupling factor, CF1. ATPase I from either algal strain was shown to be structurally distinct from high plant CF1 and to C. reinhardtii ATPases II and III.

The role of plastidic cytochrome c in algal electron transport and photophosphorylation

By an improved isolation procedure chloroplasts could be obtained from the alga Bumilleriopsis filiformis (Xanthophyceae) which exhibited high electron transport rates tightly coupled to ATP formation. Uncouplers both stimulate electron transport and inhibit photophosphorylation. These chloroplasts retain almost all soluble cytochrome c-553 besides a membrane-bound cytochrome c-554.5 (=f-554.5). Sonification or iron deficiency removed the soluble cytochrome only with a concurrent decrease of electron transport from water to methyl viologen or to NADP and decreased non-cyclic and cyclic photophosphorylation. However, photosynthetic control and the P/2e ratios remain unaltered. In Bumilleriopsis, which apparently has no plastocyanin, the soluble cytochrome c-553 seemingly links electron transport between the bound cytochrome c and P-700.

A mutant strain of Scenedesmus obliquus deficient in ribulose diphosphate carboxylase, cytochrome f and photosystem II activity

The partial reactions of photosynthesis shown by strain F208, a non-photosynthetic mutant strain of Scenedesmus obliquus, have been compared with those performed by other mutant strains which lacked; Photosystem II activity (strains 11 and F131), cytochrome f (strain 50), P-700 and cytochrome f (strain F 119), and P-700 (strains F139 and 199). In this respect the properties of strain F208 were those that would be expected if Photosystem II activity and cytochrome f were not present in this strain. Examination of the composition of strain F208 has shown the absence of cytochrome f in both the soluble and the membrane-bound form. The considerably lower level of plastoquinone compared to that found in the wild type is characteristic of the strains which lack Photosystem II activities. Fraction 1 protein could not be detected in extracts of strain F208 by sedimentation velocity experiments in the ultracentrifuge, and only 7% of the wild type ribulose diphosphate carboxylase activity was found after chromatography of these extracts on DEAE-cellulose. The properties of strain F208 are compared with those of the ac-20 and cr-1 strains of Chlamydomanas rheinhardi, both of which have a deficiency of ribulose diphosphate carboxylase which is considered to result from a deficiency of chloroplast ribosomes. Strain F208 resembles these strains in its abnormal chloroplast ultrastructure and its decreased levels of the RNA forms derived from the chloroplast ribosomes when compared with the wild type. Chloroplast fragments isolated from strains of S. obliquus which lacked cytochrome f (strains 50 and F208) were able to use diaminodurene and ascorbate as an electron donor to Photosynstem I. Since this reaction was inhibited by mercuric salts it would appear that plastocyanin, but not cytochrome f, was involved in this electron transfer.