Characterization of the Light-Induced Oxygen Gas Exchange from the IC 2 Deletion Mutant of Synechocystis PCC 6803 Lacking the Photosystem II 33 kDa Extrinsic Protein

Abstract

Abstract The absence of the extrinsic Mn-stabilizing 33 kDa protein in the IC 2 mutant of Synechocystis PCC 6803 disturbs the redox cycling of the water splitting system and retards the formation of its higher S-states (I. Vass, K. Cook, S. Deak, S. R. Mayes, and J. Barber, Biochim. Biophys. Acta 1102, 195-201 (1992)). We have performed analyses of the flashinduced oxygen exchange in the mutated cyanobacterium to clarify further the role of the 33 kDa protein. Under aerobic conditions, both the wild type and IC2 mutant show a relatively slow signal of oxygen rise on the first flash which is increased about twice by the addition of 10 μᴍ DCMU and significantly diminished by lowering the oxygen concentration in the medium. According to action spectra measurements, this mode of apparent oxygen release is mediated by PS I and can be attributed to a light induced inhibition of respiratory activity. In contrast to the wild type, having the usual oxygen evolution flash pattern with a periodicity of four, the IC2 mutant shows a binary oscillation pattern of flash-induced respiratory oxygen exchange at a flash frequency 10 Hz, being dampened with DCMU or by a lower flash frequency (< 1 Hz). Oxygen evolution due to water splitting is clearly seen in the IC2 mutant when background far-red illumination is applied to saturate the signal due to respiratory inhibition, but a quadruple oscillatory component of flash-induced oxygen evolution appears only in the presence of artificial electron acceptors under partial aerobic conditions. The mutant possesses a higher PS I/PS II ratio compared to the wild type, as judged from both the flashinduced yields and quantum efficiencies of the steady-state rates of the oxygen exchange reactions. Estimates of antenna sizes indicate about 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 rate of cyclic electron flow around PS II is enhanced. It seems that the absence of the 33 kDa protein in Synechocystis 6803 also disturbs energy transfer between adjacent PS II core complexes and may also alter their association with the phycobilisomes.