BIOPHYSICAL, BIOCHEMICAL, AND PHYSIOLOGICAL CHARACTERIZATION OF CHLAMYDOMONAS-REINHARDTII MUTANTS WITH AMINO-ACID SUBSTITUTIONS AT THE ALA(251) RESIDUE IN THE D1 PROTEIN THAT RESULT IN VARYING LEVELS OF PHOTOSYNTHETIC COMPETENCE

The Q(B) binding site of the D1 reaction center protein, located withi n a stromal loop between transmembrane helices IV and V formed by resi dues Ile(219) to Leu(272), is essential for photosynthetic electron tr ansport through photosystem II (PSII). We have examined the function o f the highly conserved Ala(251) D1 residue in this domain in chloropla st transformants of Chlamydomonas reinhardtii and found that Arg, Asp, Gin, Glu, and His substitutions are nonphotosynthetic, whereas Cys, S er, Pro, Gly, ne, Val, and Leu substitutions show various alterations in D1 turnover, photosynthesis, and photoautotrophic growth. The latte r mutations reduce the rate of Q(A) to Q(B) electron transfer, but thi s is not necessarily rate-limiting for photoautotrophic growth. The Cy s mutant divides and evolves O-2 at wild type rates, although it has s lightly higher rates of D1 synthesis and turnover and reduced electron transfer between Q(A) and Q(B). O-2 evolution, D1 synthesis, and accu mulation in the Ser, Pro, and Gly mutants in high light is reduced, bu t photoautotrophic growth rate is not affected. In contrast, the ne, V al, and Leu mutants are impaired in photoautotrophic growth and photos ynthesis in both low and high light and have elevated rates of D1 synt hesis and degradation, but D1 accumulation is normal. While rates of s ynthesis/degradation of the D1 protein are not necessarily correlated with alterations in specific parameters of PSII function in these muta nts, bulkiness of the substituted amino acids is highly correlated wit h the dissociation constant for Q(B) in the seven mutants examined. Th ese observations imply that the Ala(251) residue plays a hey role in D 1 protein.