Differential regulation of psbA and psbD gene expression, and the role of the different D1 protein copies in the cyanobacterium Thermosynechococcus elongatus BP-1

Abstract

In Thermosynechococcus elongatus BP-1, which is the preferred organism in recent structural studies of PSII, three psbA and two psbD genes code for three D1 and one D2 protein isoforms, respectively. The regulation and function of these genes and protein products is largely unknown. Therefore, we used quantitative RT-PCR to follow changes in the mRNA level of the respective genes, in combination with biophysical measurements to detect changes in the electron transport activity of Photosystem II under exposure to different visible and UV light, and temperature conditions. In cells which are acclimated to 40 micromol m(-2)s(-1) growth light conditions at 40 degrees C the main populations of the psbA and psbD transcripts arise from the psbA1 and psbD1 genes, respectively. When the temperature is raised to 60 degrees C psbA1 becomes the single dominating psbA mRNA species. Upon exposure of the cells to 500 micromol m(-2)s(-1) intensity visible light psbA3 replaces psbA1 as the dominating psbA mRNA species, and psbD2 increases at the expense of psbD1. UV-B radiation also increases the abundance of psbA3, and psbD2 at the expense of psbA1 and psbD1, respectively. From the different extent of total D1 protein loss in the absence and presence of lincomycin it was estimated that the PsbA3 protein isoform replaces PsbA1 in about 65% of PSII centers after 2 h of high light acclimation. Under the conditions of different psbA transcript distributions chlorophyll fluorescence and thermoluminescence measurements were applied to monitor charge recombination characteristics of the S2Q(A)(-) and S2Q(B)(-) states. We obtained faster decay of flash-induced chlorophyll fluorescence in the presence of DCMU, as well as lower peak temperature of the Q and B thermoluminescence bands when PsbA3 replaced PsbA1 as the main D1 protein isoform. The relevance of dynamic changes in the abundance of psbA and psbD transcript levels, as well as D1 protein isoforms in the acclimation of T. elongatus to changing environmental conditions is discussed.

Functional Characterization and Quantification of the Alternative PsbA Copies in Thermosynechococcus elongatus and Their Role in Photoprotection

“…Quantification of PsbA ProteinsQuantification of the psbA transcripts using quantitative real time PCR has become a standard method that has been successfully used for various species and growth conditions (1,17). Although this is a very powerful tool to monitor the rapid exchange between the different copies and the fast adaptation to stress conditions, the question how fast and to what extent these changes actually occur on the protein level still remains unsolved.…”

“…Fluorescence Measurements-Delayed fluorescence and flash-induced fluorescence decay measurements were performed as reported earlier (1,16).…”

An Atypical psbA Gene Encodes a Sentinel D1 Protein to Form a Physiologically Relevant Inactive Photosystem II Complex in Cyanobacteria

“…Interestingly, other alternate native D1 isoforms in various cyanobacteria investigated to date with varying expression patterns are capable of supporting PSII activity (16,18,22,60,61). In contrast, the sD1 isoform, identified in at least 30 cyanobacterial species, renders PSII inactive to protect the nitrogenase enzyme.…”

“…However, upon deletion of psbA2, levels of psbA3 transcripts increase so that the total levels of psbA transcripts remain unchanged (15). Additional psbA3 transcription can be induced upon exposure to high light and UV-B (16,17). psbA1 (slr1181) encodes a second isoform, D1Ј, which is expressed only under suboxic growth conditions (18).…”

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

“…The main difference of D1:2 from the other D1 proteins is a substitution of Gln to Glu at position 130 (Q130E). D1:2 expression is induced by HL conditions, where it reaches 70% of the total D1 proteins in T. elongatus BP-1 (Kós et al, 2008;Sander et al, 2010). Deletion mutants of the D1:2 copy show impaired growth in HL (Kulkarni and Golden, 1995;Sander et al, 2010).…”