CHARACTERIZATION OF THE LIGHT-INDUCED OXYGEN GAS-EXCHANGE FROM THE IC2 DELETION MUTANT OF SYNECHOCYSTIS PCC-6803 LACKING THE PHOTOSYSTEM-II33 KDA EXTRINSIC PROTEIN

We have performed analyses of the flash-induced oxygen exchange in the mutated cyanobacterium to clarify further the role of the 33 kDa prot ein. Under aerobic conditions, both the wild type and IC2 mutant show a relatively slow signal of oxygen rise on the first flash which is in creased about twice by the addition of 10 muM DCMU and significantly d iminished by lowering the oxygen concentration in the medium. Accordin g to action spectra measurements, this mode of apparent oxygen release is mediated by PS I and can be attributed to a light induced inhibiti on of respiratory activity. In contrast to the wild type, having the u sual oxygen evolution flash pattern with a periodicity of four, the IC 2 mutant shows a binary oscillation pattern of flash-induced respirato ry oxygen exchange at a flash frequency 10 Hz, being dampened with DCM U or by a lower flash frequency (< 1 Hz). Oxygen evolution due to wate r splitting is clearly seen in the IC2 mutant when background far-red illumination is applied to saturate the signal due to respiratory inhi bition, but a quadruple oscillatory component of flash-induced oxygen evolution appears only in the presence of artificial electron acceptor s under partial aerobic conditions. The mutant possesses a higher PS I /PS II ratio compared to the wild type, as judged from both the flash- induced yields and quantum efficiencies of the steady-state rates of t he oxygen exchange reactions. Estimates of antenna sizes indicate abou t a 20% decrease of optical cross-section at 675 nm of the PS II unit in IC2 mutants in comparison with the wild type. It is suggested that the absence of the 33 kDa protein leads to a modification of the PS II assembly and because of the slowing down of the S-state cycle, the ra te of cyclic electron flow around PS II is enhanced. It seems that the absence of the 33 kDa protein in Synechocystis 6803 also disturbs ene rgy transfer between adjacent PS II core complexes and may also alter their association with the phycobilisomes.