Revised Manuscript Received August 28, 1992 abstract: The Dl polypeptide of the photosystem II (PSII) reaction center is synthesized as a precursor polypeptide which is posttranslationally processed at the carboxy terminus. It has been shown in spinach that such processing removes nine amino acids, leaving Ala344 as the C-terminal residue [Takahashi, M., Shiraishi, T., & Asada, K.(1988) FEBS Lett. 240, 6-8; Takahashi, Y., Nakane, H., Kojima, H., & Satoh, K.(1990) Plant Cell Physiol. 31, 273-280]. We show here that processing on the carboxy side of Ala344 also occurs in the cyanobacterium Synechocystis 6803, resulting in the removal of 16 amino acids. By constructing a deletion strain of Synechocystis 6803 that lacks the three copies of the psbA gene encoding Dl, we have developed a system for generating psbA mutants. Using this system, we have constructed mutants of Synechocystis 6803 that are modified in the region of the C-terminus of the Dl polypeptide. Characterization of these mutants has revealed that (1) processing of the Dl polypeptide is blocked when the residue after the cleavage site is changed from serine toproline (mutant Ser345Pro) with the result that the manganese cluster is unable to assemble correctly;(2) the C-terminal extension of 16 amino acid residues can be deleted with little consequence either for insertion of Dl into the thylakoid membrane or for assembly of Dl into a fully active PSII complex;(3) removal of only one more residue (mutant Ala344stop) results in a loss of assembly of the manganese cluster; and (4) the ability of detergent-solubilized PSII core complexes (lacking themanganese cluster) to bind and oxidize exogenous Mn2+ by the secondary donor, Z+, is largely unaffected in theprocessing mutants (the Ser345Pro mutant of Synechocystis 6803 and the LF-1 mutant of Scenedesmus obliquus) and the truncation mutant Ala344stop. Our results are consistent with a role for processing in regulating the assembly of the photosynthetic manganese cluster and a role for the free carboxy terminus of the mature Dl polypeptide in the ligation of one or more manganese ions of the cluster.

The Dl polypeptide of the photosystem II (PSII) 1 complex plays a key role in many aspects of PSII function, from the reduction of plastoquinone to plastoquinol on the acceptor side of the complex [reviewed in Diner et al.(1991a)] to the oxidation of water to molecular oxygen on the donor side (Nixon & Diner, 1992). The primary and secondary electron-transfer reactions of PSII occur within the reaction center, a heterodimer of the Dl polypeptide and the related D2