Biophysical, biochemical, and physiological characterization of Chlamydomonas reinhardtii mutants with amino acid substitutions at the Ala super(251) residue in the D1 protein that result in varying levels of photosynthetic competence

• Published in: The Journal of biological chemistry 1998-05, Vol.273 (18), p.11082-11091
• Main Authors: Lardans, A, Foerster, B, Prasil, O, Falkowski, P G, Sobolev, V, Edelman, M, Osmond, C B, Gillham, N W, Boynton, JE
• Format: Article
• Language: English
• Subjects: Chlamydomonas reinhardtii; Freshwater
• ISSN: 0021-9258

The Q sub(B) binding site of the D1 reaction center protein, located within a stromal loop between transmembrane helices IV and V formed by residues Ile super(219) to Leu super(272), is essential for photosynthetic electron transport through photosystem II (PSII). We have examined the function of the highly conserved Ala super(251) D1 residue in this domain in chloroplast transformants of Chlamydomonas reinhardtii and found that Arg, Asp, Gln, Glu, and His substitutions are nonphotosynthetic, whereas Cys, Ser, Pro, Gly, Ile, Val, and Leu substitutions show various alterations in D1 turnover, photosynthesis, and photoautotrophic growth. The latter mutations reduce the rate of Q sub(A) to Q sub(B) electron transfer, but this is not necessarily rate-limiting for photoautotrophic growth. The Cys mutant divides and evolves O sub(2) at wild type rates, although it has slightly higher rates of D1 synthesis and turnover and reduced electron transfer between Q sub(A) and Q sub(B). O sub(2) evolution, D1 synthesis, and accumulation in the Ser, Pro, and Gly mutants in high light is reduced, but photoautotrophic growth rate is not affected. In contrast, the Ile, Val, and Leu mutants are impaired in photoautotrophic growth and photosynthesis in both low and high light and have elevated rates of D1 synthesis and degradation, but D1 accumulation is normal. While rates of synthesis/degradation of the D1 protein are not necessarily correlated with alterations in specific parameters of PSII function in these mutants, bulkiness of the substituted amino acids is highly correlated with the dissociation constant for Q sub(B) in the seven mutants examined. These observations imply that the Ala super(251) residue plays a key role in D1 protein.