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Volume 272, Number 1, Issue of January 3, 1997 pp. 210-216
©1997 by The American Society for Biochemistry and Molecular Biology, Inc.

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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

(Received for publication, July 3, 1996, and in revised form, October 17, 1996)

From the Developmental, Cell, and Molecular Biology Group, Departments of Botany and Zoology, Duke University, Durham, North Carolina 27708-1000

In Cyanobacteria and Chlamydomonas reinhardtii, substitution of valine for alanine at position 251 of the photosystem II D1 protein in the loop between transmembrane helices IV and V confers resistance to herbicides that reduce photosystem II function and increases sensitivity to photoinhibition. Using site-directed mutagenesis and chloroplast transformation in Chlamydomonas we have examined further the role of residue 251 in relation to D1 structure, function, and photosynthetic performance. Of the 12 different amino acid substitutions for Ala²⁵¹ introduced at this position, five (Arg, Asp, Gln, Glu, and His) resulted in a nonphotosynthetic phenotype. Transformants with the Arg²⁵¹ substitution synthesize a normal sized 32-kDa D1 protein with greatly reduced stability. The Gln, Glu, His, and Asp transformants make a 33-34-kDa form of the D1 protein of varying stability as well as an immunologically related polypeptide of 24-25 kDa corresponding to the N-terminal portion of D1 that is unstable and appears to be an aborted D1 translation product. All mutant forms of the D1 protein are intrinsic to the 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 Ala²⁵¹ has a key role in the structure and function of the IV-V loop region.

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