SITE-DIRECTED MUTATIONS AT RESIDUE-251 OF THE PHOTOSYSTEM-II D1 PROTEIN OF CHLAMYDOMONAS THAT RESULT IN A NONPHOTOSYNTHETIC PHENOTYPE AND IMPAIR D1 SYNTHESIS AND ACCUMULATION
A. Lardans et al., SITE-DIRECTED MUTATIONS AT RESIDUE-251 OF THE PHOTOSYSTEM-II D1 PROTEIN OF CHLAMYDOMONAS THAT RESULT IN A NONPHOTOSYNTHETIC PHENOTYPE AND IMPAIR D1 SYNTHESIS AND ACCUMULATION, The Journal of biological chemistry, 272(1), 1997, pp. 210-216
In Cyanobacteria and Chlamydomonas reinhardtii, substitution of valine
for alanine at position 251 of the photosystem II D1 protein in the l
oop between trans membrane helices IV and V confers resistance to herb
icides that reduce photosystem II function and increases sensitivity t
o photoinhibition. Using site-directed mutagenesis and chloroplast tra
nsformation in Chlamydomonas we have examined further the role of resi
due 251 in relation to D1 structure, function, and photosynthetic perf
ormance. Of the 12 different amino acid substitutions for Ala(251) int
roduced at this position, five (Arg, Asp, Gln, Glu, and His) resulted
in a nonphotosynthetic phenotype. Transformants with the Arg(251) subs
titution synthesize a normal sized 32-kDa D1 protein with greatly redu
ced stability. The Gln, Glu, His, and Asp transformants make a 33-34-k
Da form of the D1 protein of varying stability as well as an immunolog
ically related polypeptide of 24-25 kDa corresponding to the N-termina
l portion of D1 that is unstable and appears to be an aborted D1 trans
lation product. All mutant forms of the D1 protein are intrinsic to th
e thylakoids. In contrast to previous studies in Cyanobacteria showing
that residues in the IV-V loop can be mutated or deleted without loss
of photosynthetic competence, our results suggest that Ala(251) has a
key role in the structure and function of the IV-V loop region.