Biology subjects

Kappel, S.

Publications and source records attributed to Kappel, S..

3 recordsLinked to original sources

bioRxiv2023

A congestion downstream of PSI causes the over-reduction of the electron transport chain in pgr5 independent of membrane energization

The thylakoid protein Proton Gradient Regulation5 (PGR5) is thought to be a key component of cyclic electron flux around photosystem I. The pgr5 mutant is characterized by impaired proton motive force (pmf) formation across the thylakoid membrane, decreased photoprotective non-photochemical quenching (NPQ), and an over-reduction of the PSI acceptor side. This over-reduction has been attributed to impaired photosynthetic control, which down-regulates plastoquinol re-oxidation at the cytochrome b6f complex when the lumen is strongly acidified. Here, using the cgl160 ATP synthase assembly mutant, we show that in cgl160 pgr5 double mutants, both the pmf across the thylakoid membrane and NPQ are fully restored to wild-type levels. However, the acceptor-side limitation of PSI in the double mutants stays comparable to the single pgr5 mutant. This demonstrates that impaired photosynthetic control is not causal for the over-reduction of the PSI acceptor side in pgr5. Instead, we show that both in pgr5 and the clg160 pgr5 mutants, the entire high-potential chain from cytochrome f to PSI remains strongly reduced in high light. This leads to insufficient oxidizing power for plastoquinol re-oxidation by the cytochrome b6f complex, thus impairing pmf formation. We conclude that PGR5 plays a critical role in electron partitioning downstream of PSI.

plant biology↗

bioRxiv2023

A eukaryote-specific factor mediates an early step in the assembly of plant photosystem II

ABSTRACTThe initial step of oxygenic photosynthesis is the thermodynamically challenging extraction of electrons from water and the release of molecular oxygen. This light-driven process, which is the basis of life on Earth, is catalyzed by the photosystem II (PSII) within the thylakoid membrane of photosynthetic organisms. The biogenesis of PSII requires a controlled step-wise assembly process of which the early steps are considered to be highly conserved between plants and their cyanobacterial progenitors. This assembly process involves auxiliary proteins, which are likewise conserved. In the present work, we show that in plants, the early assembly step, in which the PSII reaction center (RC) is associated with the intrinsic antenna protein CP47 to form the RC47 intermediate, is facilitated by a novel eukaryote-exclusive assembly factor. This factor, we named DEAP2 for DECREASED ELECTRON TRANSPORT AT PSII, works in concert with the conserved PAM68 assembly factor. The deap2 and pam68 mutants showed similar defects in PSII accumulation and assembly of the RC47 intermediate. The combined lack of both proteins results in a loss of functional PSII and the inability of plants to grow photoautotrophically on soil. While overexpression of DEAP2 partially rescued the pam68 PSII accumulation phenotype, this effect was not reciprocal. DEAP2 accumulates at 20-fold higher levels than PAM68, together suggesting that both proteins have distinct functions. In summary, our results uncover eukaryotic adjustments to the PSII assembly process, which involve the addition of DEAP2 for the rapid progression from RC to RC47.

plant biology↗

bioRxiv2023

Cracking the floral quartet code: How do multimers of MIKC C-type MADS-domain transcription factors recognize their target genes?

MADS-domain transcription factors (MTFs) are involved in the control of many important processes in eukaryotes. They are defined by the presence of a unique and highly conserved DNA-binding domain, the MADS-domain. MTFs bind to double-stranded DNA as dimers and recognize specific sequences termed CArG-boxes (such as 5-CC(A/T)6GG-3) and similar sequences that occur hundreds of thousand times in a typical flowering plant genome. The number of MTF-encoding genes increased by about two orders of magnitude during land plant evolution, resulting in roughly about 100 genes in flowering plant genomes. This raises the question as to how dozens of different, but highly similar MTFs accurately recognize the cis-regulatory elements of diverse target genes when the core binding sequence (CArG-box) occurs at such a high frequency. Besides the usual processes, such as base and shape readout of individual DNA sequences by dimers of MTFs, an important sublineage of MTFs in plants, termed MIKCC-type MTFs (MC-MTFs) has evolved an additional mechanism to increase the accurate recognition of target genes: the formation of heterotetramers of closely related proteins that bind to two CArG-boxes on the same DNA strand involving DNA-looping. MC-MTFs control important developmental processes in flowering plants, ranging from root and shoot to flower, fruit and seed development. The way MC-MTFs bind to DNA and select their target genes is hence not only of high biological interest, but also of great agronomic and economic importance. In this article we review the interplay of the different mechanisms of target gene recognition, from the ordinary (base readout) via the extravagant (shape readout) to the idiosyncratic (recognition of the distance and orientation of two CArG-boxes by heterotetramers of MC-MTFs). A special focus of our treatment is on the structural prerequisites of MC-MTFs that enable the specific recognition of target genes.

genetics↗