Two functionally distinct forms of the photosystem II reaction-center protein D1 in the cyanobacterium Synechococcus sp. PCC 7942.

Abstract

The cyanobacterium Synechococcus sp. PCC 7942 possesses a small psbA multigene family that codes for two distinct forms of the photosystem II reaction-center protein Dl (D1:1 and D1:2). We showed previously that the normally predominant Dl form (D1:1) was rapidly replaced with the alternative D1:2 when cells adapted to a photon irradiance of 50 pmol mM2's'1 are shifted to 500 ,umol m-2 s-1 and that this interchange was readily reversible once cells were allowed to recover under the original growth conditions. By using thepsbA inactivation mutants R2S2C3 and R2K1 (which synthesize only D1:1 and D1:2, respectively), we showed that this interchange between Dl forms was essential for limiting the degree of photoinhibition as well as enabling a rapid recovery of photosynthesis. In this report, we have extended these findings by examining whether any intrinsic functional differences exist between the two Dl forms that may afford increased resistance to photoinhibition. Initial studies on the rate of Dl degradation at three photon irradiances (50, 200, and 500 #mol'm-2 s'1) showed that the rates ofdegradation for both Dl forms increase with increasing photon flux density but that there was no signiicant difference between Dl:1 and D1:2. Analysis of light-response curves for oxygen evolution for the mutants R2S2C3 and R2K1 revealed that cells with photosystem II reaction centers containing D1:2 have a higher apparent quantum yield (=25%) than cells possessing D1:1. Further studies using chlorophyll a fluorescence measurements confirmed that R2K1 has a higher photochemical yield than R2S2C3; that is, a more efficient conversion of excitation energy from photon absorption into photochemistry. We believe that the higher photochemical efficiency of reaction centers containing D1:2 is causally related to the preferential induction of D1:2 at high light and thus may be an integral component of the protection mechanism within Synechococcus sp. PCC 7942 against photoinhibition.The reaction center of photosystem II (PSII) consists of two structurally similar proteins, Dl and D2, as well as cytochrome b559 and the psbl gene product (for a recent review, see ref.

Differences in Responses to Darkness betweenMicrocystis aeruginosaandChlorella pyrenoidosa

“…In cyanobacteria, changes in F, : F, under conditions of constant pigment content correlate well with changes in independent measurements of PS I1 function such as oxygen evolution , Clarke et al 1993, Luettge et al 1995, but the absolute level of F, : F, is not a reliable indicator of PS I1 function. In the present study, the Qo decrease in M. aeruginosa was more significant than that of C. pyrenoidosa under dark condition, which reflected clearly that inhibition of M aeruginosa PSI1 function was more affected by darkness.…”

Flavodiiron Protein Flv2/Flv4-Related Photoprotective Mechanism Dissipates Excitation Pressure of PSII in Cooperation with Phycobilisomes in Cyanobacteria

“…Deletion mutants of the D1:2 copy show impaired growth in HL (Kulkarni and Golden, 1995;Sander et al, 2010). In fact, the exchange of D1 protein forms with a preference for D1:2 in HL provides increased phototolerance to the cells (Krupa et al, 1991;Clarke et al, 1993), which is accompanied by accelerated nonradiative charge recombination, with a consequent reduced production of singlet oxygen (Tichý et al, 2003;Cser and Vass, 2007;Sander et al, 2010).…”

Tansley Review No. 117

“…By contrast, when shade-adapted species of old growth forests were exposed to high irradiances in their desiccated state, they displayed both significant photoinhibition without complete recovery after 3 d and extensive Chl a depletion (Gauslaa & Solhaug, 1996. Lichens with cyanobacterial photobionts might be particularly susceptible to photoinhibition, first because cyanobacteria lack the zeaxanthin-violaxanthin cycle (Demmig-Adams et al, 1990a,b), and second because their PSII reaction-centre protein, D1, has an inherently lower resistance to photoinhibition (Clarke et al, 1993). So far, however, the biophysical characteristics of PSII reaction-centre proteins have not been investigated for any lichenized cyanobacterium .…”