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
A. Lardans et al., 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 Journal of biological chemistry, 273(18), 1998, pp. 11082-11091
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.