Trends in Genetics
Volume 37, Issue 3, March 2021, Pages 211-215
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Tightening the Screws on PsbA in Cyanobacteria

https://doi.org/10.1016/j.tig.2020.08.018Get rights and content

Cyanobacterial genomes encode several isoforms of the D1 (PsbA) subunit of Photosystem II (PSII). The distinct regulation of each isoform ensures adaptation under changing environmental conditions. Uncovering the missing elements of signal transduction pathways and psbA gene expression could open new avenues in engineering programs of cyanobacterial strains.

Section snippets

The Relevance of psbA Regulation in Cyanobacteria

PSII is the water-splitting (oxygen-evolving) enzyme of photosynthesis. Among PSII core proteins, the D1 subunit (encoded by the psbA gene) provides most of the ligands for the water-splitting cluster involved in primary charge separation and evolution of molecular oxygen. This oxidative chemistry causes excessive light-induced photodamage to the D1 subunit than to any other core protein. Therefore, the expression of D1 protein is stringently regulated in photoautotrophs and exhibits a high

Sigma-Factor-Mediated Regulation of psbA Gene Expression

Specific promoter recognition by RNA polymerase and transcription initiation relies on sigma factor proteins in prokaryotes [3., 4., 5., 6.]. The expression of cyanobacterial psbA genes and their light-responsive nature is regulated by the housekeeping sigma factor and alternative sigma factors respectively (Figure 1A). Mutants of Synechocystis lacking sigma factor SigD and SigB display considerably less accumulation of psbA2/psbA3 transcripts after 45 min of high light exposure, whereas the

D1 Regulation through Response Regulators and Histidine Kinases

The expression of photosynthetic genes is believed to be regulated by an altered redox state of the electron transport chain, perceived by sensory regulators in cyanobacteria. The putative histidine kinase, NblS, is one such regulator that is found to control the expression of psbA genes by a blue/UV-light and high-light stimulus in Synechococcus [8]. The deletion of nblS gene in Synechococcus results in the elimination of light-mediated regulation of all psbA genes [9]. Interestingly, a PAS

Impact of Transcriptional and Post-transcriptional Effectors on psbA Expression

The cmpABCD operon, encoding a high-affinity bicarbonate transporter, is induced under high-light and low-CO2 conditions by a LysR family protein, CmpR in Synechococcus [14]. The binding sites of CmpR are also present in the promoter regions of the genes, psbAII and psbAIII, and shown to be responsive to high light and low CO2. However, upon loss of cmpR, both genes are still induced under the above conditions, suggesting the involvement of more than one mechanism and additional transcriptional

Concluding Remarks

Being the most iconic subunit of photosystems, D1 has been studied extensively, resulting in the accumulation of a large amount of data regarding its structure, function, and evolution. Two recent findings concerning D1 proteins are breakthrough in contemporary photosynthetic research: (i) the role of a highly divergent D1 in chlorophyll f biosynthesis in cyanobacteria equipped with a sophisticated FarLiP (far-red light photoacclimation) gene cluster; and (ii) improved heat resistance by a

Acknowledgments

We are grateful for all the colleagues whose work is cited or could not be cited in this Forum. We extend our apology to the authors whose important work is not mentioned here owing to word and citation constraints. We are thankful to Dr Michael Reppert (Purdue University, USA) for reading the manuscript and providing useful comments. The infrastructural support from the Department of Science and Technology, New Delhi, Govt. of India, through FIST grant (Grant No. 1196 SR/FST/LS-I/ 2017/4) and

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