Chlorophyll in a Synechocystis sp. PCC 6803 mutant without photosystem I and photosystem II core complexes: Evidence for peripheral antenna chlorophylls in cyanobacteria

The chlorophyll protein organization has been investigated in thylakoid membranes from mutants of the cyanobacterium Synechocystis sp. PCC 6803, in which the photosystem II (PS II) genes psbB and/or psbC (coding for CP47 and CP43, respectively) were inactivated together with the psaAB operon (coding for the photosystem I (PS I) core complex) and the apcE gene (coding for the phycobilisome anchor protein). Lack of the CP43 protein led to a significant decrease of the D1, D2, and CP47 proteins and a decrease in the 77 K fluorescence emission peak at 685 nm. In the absence of the CP47 protein, no PS II reaction center assembly was detected and the 77 K fluorescence emission peak at 695 nm was lost. The psbB^-/psbC^-/PS I-less/apcE^- mutant had no assembly of the D1, D2, CP47, and CP43 proteins, had lost the 77 K fluorescence emission peaks at 685 and 695 nm, but retained about 15% of the chlorophyll present in the PS I-less/apcE^- background strain. A broad 77 K fluorescence emission band with a maximum at 678 nm was displayed in the PS II-less, PS I-less mutant upon excitation of the remaining chlorophyll. A 678 nm shoulder was observed in the 77 K fluorescence emission spectrum of thylakoids from the psbB^-/PS I-less/apcE^- mutant, which still contains CP43 but no PS II reaction center. This shoulder was absent in thylakoids from the psbC^-/PS I-less/apcE^- mutant, which contain some PS II reaction center complexes. These results are consistent with the chlorophyll associated with the 678 nm emission to serve as peripheral antenna to PS II. The fluorescence emission characteristics of this chlorophyll are different from those of an accessory chlorophyll-binding protein expressed under iron-stress conditions in cyanobacteria. The chlorophyll remaining in the absence of PS II and PS I is indicative of a new chlorophyll-binding protein in cyanobacterial thylakoids.