Activation of the silent psbA1 gene in the cyanobacterium Synechocystis sp strain 6803 produces a novel and functional D1 protein.

The photosystem II reaction center protein D1 in Synechocystis sp strain 6803 is encoded by the psbA2 and psbA3 genes of the three-membered psbA gene family. The silent and divergent psbA1 copy of the psbA gene family was activated by exchanging part of its upstream region with a corresponding fragment of the psbA2 copy. The light-regulated expression of the activated psbA1 gene showed that the inserted psbA2 segment contains the information necessary for light-dependent as well as high-light-stimulated transcription. The activated psbA1 gene expressed a novel D1 protein, D1'. A mutant strain containing psbA1 as the only active psbA gene grew photoautotrophically at a rate comparable to that of the wild type. This finding demonstrates that despite its unusual amino acid sequence, D1 is exchangeable for D1 in the photosystem II complex, at least under normal laboratory conditions. The D1' protein was found to have a degradation rate similar to that of the D1 protein under low- or high-light conditions. Another mutant containing the activated psbA1 gene together with the psbA2 and psbA3 genes produced both the D1 and D1' proteins.