Thylakoids of genetically engineered species of Synechococcus PCC7942 were investigated by thermoluminescence (TL), polarographic and herbicide-binding studies. The results were as follows: 1) Deletion of either copy I or copy II/III of the psbA gene family resulted in modified oscillation patterns of flash induced TL and O2 evolution compared to wild type cells. 2) Replacement of Ser264 in psbA I by Ala or Gly inducing strong herbicide resistance leads to a downshift of the TL peak temperature of B-band by 10 C and alters the oscillation pattern of flash induced TL and O2 evolution significantly to exhibit a pattern with comparatively high O2 yield after the 2nd flash. Additional deletion of psbA II III does not lead to significant changes compared to the single mutated strains. 3) Contrary to Ser,M mutants, replacement of Phe255 by Tyr in psbA I alone, which also induces herbicide resistance, is not accompanied by- corresponding changes in TL peak temperature and/or oscillation pattern of flash-induced TL and O2 evolution. However, strongly altered properties are observed after additional deletion of psbA II/III. Based on these data it is inferred that expression of either psbA I or psbA II III genes only gives rise to PS II complexes slightly different in their function. Site-directed muta- genesis of psbA I gene modulates the expression ratio between copies I and II/III. The possible implications are discussed.
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