Consists of one review article and eleven research articles.
Two papers examine the consequences of oxidative damage by Reactive Oxygen Species (ROS) within the photosystem. Pospíšill et al. (https://doi.org/10.1007/s11120-022-00922-x) present a review examining the role of ROS in oxidative signal transduction. The authors argue that since ROS lifetimes (and, consequently, diffusion distances) are typically short, direct retrograde signaling (chloroplast to nucleus) by ROS is unlikely. They hypothesize that oxidatively modified lipids (and possibly pigments) have longer lifetimes, and may be involved in retrograde signaling. The possibility that oxidatively modified peptides derived from the proteolysis of oxidatively damaged proteins may also be involved is also discussed. In a research article by Kale et al. (https://doi.org/10.1007/s11120-022-00902-1), differential oxidative modification, probably by 1O2, of Lhcb1 and Lhcb2 proteins associated with either spinach PS II membranes or PS I-LHC I-LHC II membranes was documented. The authors concluded that, in large measure, different populations of LHC II trimers were associated with PS II and PS I.
Bielczynski et al. (https://doi.org/10.1007/s11120-022-00907-w) examine the principal component of Non-Photochemical Quenching (NPQ), qE. It had been hypothesized that during the formation of qE, significant structural reorganization of the PS II supercomplex occurred which was reversed during qE relaxation (Holzwarth et al. 2009; Huang et al. 2021). Using NPQ-competent thylakoids which exhibited qE formation and relaxation, the authors demonstrated that no large structural changes occurred in the C2S2M2 and C2S2M supercomplexes.
Two papers examine manganese assimilation into the active site for water oxidation. Russell and Vinyard (https://doi.org/10.1007/s11120-021-00886-4) have used dual-mode EPR spectroscopy to study the earliest steps of photo-assembly of the PS II oxygen-evolving complex. At physiologically relevant pH values for the thylakoid lumen, they show that chloride is an essential cofactor for initial Mn(II) binding and for a deprotonation event that facilitates Mn(III) formation. Mino and Asada (https://doi.org/10.1007/s11120-021-00885-5) have identified two high affinity Mn(II) binding sites using pulsed EPR methods that shed light on the mechanism of OEC photo-assembly. Conditions were optimized to either leave one Mn(II) behind during mild depletion or to add one Mn(II) back to fully depleted samples. Their results suggest that the binding of extrinsic subunits during OEC photo-assembly changes Mn(II) binding affinity, with key implications for a proposed metal translocation model.
Two papers examine the PS II assembly factor Psb27 in cyanobacteria. Johnson et al. (https://doi.org/10.1007/s11120-021-00895-3) examine its function in Synechocystis sp. PCC 6803. These investigators examined a Psb27 deletion strain, its genetic complement, a Psb27 overexpression line, and wild type using a variety of biochemical and biophysical probes. The authors demonstrated that Psb27 elicits efficient dissipation of excitation energy preventing photodamage to pre-PS II assembly and repair complexes. Lambertz, et al. (https://doi.org/10.1007/s11120-021-00891-7), using high-resolution mass spectrometry, studied the N-terminal lipid modification of Psb27 in Thermosynechococcus elongatus BP-1. This modification is not resolved in the current cryo-EM structures of assembly complexes (Zabret et al. 2021; Knoppová et al. 2014). A total of six (!) different lipid isoforms were identified. Differential functions (if any) for these isoforms have not been determined at this time and their elucidation may provide a platform for elucidating the function(s) of N-terminal lipid modifications, in general.
Konert, et al. (https://doi.org/10.1007/s11120-022-00904-z) examined the four High-Light-Inducible proteins (Hlips) in the cyanobacterium Synechocystis sp. PCC 6803, HliA-D. These proteins are essential for survival under stress conditions. These investigators demonstrated that HliA and HliB form heterodimers with HliC. Earlier investigations (Knoppová et al. 2014) had demonstrated the heterodimeric association of HliD with HliC. The authors present a hypothesis that these heterodimers associate with the CP47 assembly module and later CP47-containing assembly intermediates, allowing the thermal dissipation of excitation energy during PS II assembly.
Rahimzadeh-Karvansara et al. (https://doi.org/10.1007/s11120-022-00908-9) introduce Psb34 as a regulatory component of PS II assembly in Synechocystis sp. PCC 6803. Psb34 binds to CP47-containing PS II intermediates competitively with high-light-inducible proteins (Hlips). A model is proposed in which Hlips bind to CP47 modules in early assembly steps and are recycled by their later replacement with Psb34.
The function of the small subunit PsbJ is explored in a work by Boussac and coworkers (https://doi.org/10.1007/s11120-021-00880-w). Using a psbJ knockout strain of Thermosynechococcus elongatus and a diverse set of biochemical and biophysical techniques, they conclude that PsbJ modulates the reduction potential of QB/QB−. Intriguingly, the predicted thermodynamic change would protect maturing PS II from photoinhibition and favor OEC photo-assembly.
Beckova et al. (https://doi.org/10.1007/s11120-022-00896-w) argue against the existence of a “no reaction center” intermediate of PS II assembly, which had previously been suggested (Weisz Daniel et al. 2019). This provocative work demonstrates that CP47 and CP43 intermediate modules often comigrate in native electrophoresis and ultracentrifugation experiments. By tagging either CP43 or CP47, they show that the two modules are distinct, and no combined intermediate is observed.
The paper by Havurinne et al. (https://doi.org/10.1007/s11120-021-00883-7) presents a characterization of the mechanisms by which sea slugs protect the intact plastids they acquire from Acetabularia by the process known as kleptoplasty. These fully functional intact chloroplasts are protected from photoinhibition in one of two ways according to the authors’ data. In the first way, tight packing of the plastids in the slugs occurs so that plastids in the outer layer of a cluster protect the plastids buried more deeply in the cluster. The second way utilizes screening of photoinhibitory UV light that is absorbed by the mucous and skin of the slug itself.