Transcript Levels and Synthesis of Photosystem II Components in Cyanobacterial Mutants with Inactivated Photosystem II Genes.

After interruption or deletion of the photosystem II genes psbB, psbC, and psbD in the cyanobacterium Synechocystis sp. PCC 6803, thylakoids from such mutants were found to be depleted in a number of photosystem II proteins in addition to those for which the gene(s) had been inactivated. Transcript levels of photosystem II genes were measured and protein pulse-labeling was carried out to determine the reason for this effect. Transcripts of all photosystem II genes except the inactivated one(s) were found to be present in the various mutants. In certain cases, inactivation of one photosystem II gene led to overexpression of another. Protein pulse-labeling experiments using 35S-methionine, in which not only the rapidly turning over D1 protein but also D2, CP43, and CP47 appear to be preferentially labeled, showed that the mutants studied synthesize the D1 protein as well as other photosystem II proteins whose genes were not inactivated. The fact that, in the various mutants, photosystem II proteins for which the gene is not inactivated are synthesized but do not accumulate in the thylakoid indicates that the psbB, psbC, and psbD gene products are all required for a stable assembly of the photosystem II complex.