Photosystem stoichiometry and state transitions in a mutant of the cyanobacterium Synechococcus sp PCC 7002 lacking phycocyanin

Phycobilisomes (PBS) function as light-harvesting antenna complexes in cyan obacteria, red algae and cyanelles. They are composed of two substructures: the core and peripheral rods. Interposon mutagenesis of the cpcBA genes of Synechococcus sp. PCC 7002 resulted in a strain (PR6008) lacking phycocyan in and thus the ability to form peripheral rods. Difference absorption spec troscopy of whole cells showed that intact PBS cores were assembled in vivo in the cpcBA mutant strain PR6008. Fluorescence induction measurements dem onstrated that the PBS cores are able to deliver absorbed light energy to p hotosystem (PS) II, and fluorescence induction transients in the presence o f DCMU showed that PR6008 cells could perform a state 2 to state 1 transiti on with similar kinetics to that of the wild-type cells. Thus, PBS core ass embly, light-harvesting functions and energy transfer to PS I were not depe ndent upon the assembly of the peripheral rods. The ratio of PS II:PS I in the PR6008 cells was significantly increased, nearly twice that of the wild -type cells, possibly a result of longterm adaptation to compensate for the reduced antenna size of PS II. However, the ratio of PBS cores:chlorophyll remained unchanged. This result indicates that approximately half of the P S II reaction centers in the PR6008 cells had no closely associated PBS cor es. (C) 2001 Published by Elsevier Science B.V.