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Functional modes of Photosystem II under photomixotrophy in distinct variants of Synechocystis sp. PCC 6803

ABSTRACT: The regulation of photosynthetic electron transport during photomixotrophic growth in cyanobacteria remains incompletely understood. In this study, we characterized four wild-type strains (WT1–WT4) of Synechocystis sp. PCC 6803 and observed distinct strain-specific differences in Photosystem II (PSII) function under photomixotrophic conditions. Specifically, WT1 and WT2 exhibited near-complete inhibition of electron transfer from QA⁻ to QB following approximately three days of glucose supplementation, resulting in a metabolic shift toward photoheterotrophy. Despite this inhibition, both strains showed improved growth compared to photoautotrophic conditions. Observed electron transport blockade correlated with increased accumulation of the PSII-associated protein Psb28-2, and an elevated PSII dimer-to-monomer ratio in WT1 and WT2. Computational modeling supports a role for Psb28-2 in limiting forward electron flow and stabilizing PSII dimers, potentially enhancing structural integrity and facilitating maintenance of the photosynthetic apparatus. This stabilization may be driven by a transient downregulation of linear electron transport to prevent over-reduction of the electron transport chain. In contrast, WT3 and WT4 maintained photomixotrophic growth throughout the experiment but exhibited slower growth rates than WT1 and WT2. Although glucose uptake was reduced in WT1 and WT2, both strains accumulated more glycogen than WT3 and WT4, suggesting divergent regulation of carbon allocation and storage metabolism. These findings highlight the capacity of cyanobacterial strains to deploy distinct metabolic strategies to optimize photosynthetic function, carbon assimilation, and energy storage under photomixotrophic conditions.

INSTRUMENT(S):

ORGANISM(S): Synechocystis Sp. (strain Pcc 6803 / Kazusa) Bacteria

SUBMITTER: Emil Sporre

LAB HEAD: Yagut Allahverdiyeva-Rinne

PROVIDER: PXD066779 | Pride | 2026-01-02

REPOSITORIES: Pride

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			Action	DRS
	20240323_pcc6803.elib.peptides.txt	Txt
	20240323_pcc6803.elib.proteins.txt	Txt
	HF_20240423_DIA_350to1000_10mz_40min_1to32_700nLmin_60C_2kV_2uL_BA1.mzML	Mzml
	HF_20240423_DIA_350to1000_10mz_40min_1to32_700nLmin_60C_2kV_2uL_BA1.raw	Raw
	HF_20240423_DIA_350to1000_10mz_40min_1to32_700nLmin_60C_2kV_2uL_BA2.mzML	Mzml

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Project description:During Zea mays (maize) C4 differentiation, mesophyll (M) and bundle sheath (BS) cells accumulate distinct sets of photosynthetic enzymes, with very low photosystem II (PSII) content in BS chloroplasts. Consequently, there is little linear electron transport in the BS and ATP is generated by cyclic electron flow. In contrast, M thylakoids are very similar to those of C3 plants and produce the ATP and NADPH that drive metabolic activities. Regulation of this differentiation process is poorly understood but involves expression and coordination of nuclear and plastid genomes. Here, we identify a recessive allele of the maize Hcf136 homologue that in Arabidopsis thaliana functions as a PSII stability or assembly factor located in the thylakoid lumen. Proteome analysis of the thylakoids and electron microscopy reveal that Zm hcf136 lacks PSII complexes and grana thylakoids in M chloroplasts, consistent with the previously defined Arabidopsis function. Interestingly, hcf136 is also defective in processing the full-length psbB-psbT-psbH-petB-petD polycistron specifically in M chloroplasts. To determine whether the loss of PSII in M cells affects C4 differentiation, we performed cell-type specific transcript analysis of hcf136 and wild-type seedlings. The results indicate that M and BS cells respond uniquely to the loss of PSII, with little overlap in gene expression changes between data sets. These results are discussed in the context of signals that may drive differential gene expression in C4 photosynthesis. To explore the disruption of PSII activity on gene expression, transcript profiles from separated M and BS cells were examined using two-label microarray analysis. Total RNA was isolated from the second leaves of mutant and wild-type silbings. Six biological replicates were used to compare wild-type and mutant transcript profiles in separate M and BS experiments. To maximize biological replication, different seedling pools were used for each of the 12 hybridizations. Microarray experiments and analyses were performed using the Genisphere MPX900 kit and the Maize Array Consortium oligonucleotide platform (GPL5439; GPL5440). Feature intensity values were log-transformed and corrected for local background signal, and a LOWESS procedure (Dudoit et al., 2002) was used to normalize between channels. Features with either low or saturating signal intensity were discarded from further analysis. High expression filtering was less stringent to avoid elimination of previously characterized, high abundance, C4 cell-specific transcripts. After filtering, features that were not assigned an MZ number by the Maize Array Consortium were discarded from further analysis. The moderated t-test (Smyth, 2004) using the R package limma was applied to identify differentially expressed genes. The p-values for each test (gene) were converted to q-values for false discovery rate analysis as described by Storey et al. (2004). To avoid confounding treatment effects associated with direct comparisons of M and BS transcriptomes (Sawers et al., 2007), comparisons were only made using the same cell type across the hcf136 and wild-type sibling genotypes. Bundle sheath (BS) Samples: GSM245063-GSM245164 Mesophyll (M) Samples: GSM245165 - GSM245206

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Functional modes of Photosystem II under photomixotrophy in distinct variants of Synechocystis sp. PCC 6803

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