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Most cyanobacterial genomes possess more than two copies of genes encoding cyAbrBs (cyanobacterial AbrB-like proteins) having an AbrB-like DNA-binding domain at their C-terminal region. Accumulating data suggest that a wide variety of metabolic and physiologic processes are regulated by cyAbrBs. In this study, we investigated the function of the essential gene cyabrB1 (sll0359) in Synechocystis sp. PCC 6803 by using CRISPR interference technology. The conditional knockdown of cyabrB1 caused increases of cyAbrB2 transcript and protein levels. However, the effect of cyabrB1 knockdown on global gene expression profile was quite limited compared to the previously reported profound effect of knockout of cyabrB2. Among 24 up-regulated genes, 16 genes were members of the divergently transcribed icfG and sll1783 operons related to carbon metabolism. The results of this and previous studies indicate the different contributions of two cyAbrBs to transcriptional regulation of genes related to carbon, hydrogen and nitrogen metabolism. Possession of a pair of cyAbrBs has been highly conserved during the course of evolution of the cyanobacterial phylum, suggesting physiological significance of transcriptional regulation attained by their interaction.

Phycobilisomes (PBSs) are photosynthetic light-harvesting antennae and appear to be loosely bound to photosystem I (PSI). We previously found unique protein bands in each PSI fraction in heterocysts of Anabaena sp. PCC 7120 by two-dimensional blue native/SDS-PAGE; however, the protein bands have not been identified. Here we analyzed the protein bands by mass spectrometry, which were identified as CpcL, one of the components in PBSs. As different composition and organization of Anabaena PSI-PBS supercomplexes were observed, the expression and binding properties of PBSs including CpcL to PSIs in this cyanobacterium may be diversified in response to its living environments., Competing Interests: The authors declare that there are no conflicts of interest present., (Copyright: © 2024 by the authors.)

Iron-stress-induced-A proteins (IsiAs) are expressed in cyanobacteria under iron-deficient conditions. The cyanobacterium Anabaena sp. PCC 7120 has four isiA genes; however, their binding property and functional roles in PSI are still missing. We analyzed a cryo-electron microscopy structure of a PSI-IsiA supercomplex isolated from Anabaena grown under an iron-deficient condition. The PSI-IsiA structure contains six IsiA subunits associated with the PsaA side of a PSI core monomer. Three of the six IsiA subunits were identified as IsiA1 and IsiA2. The PSI-IsiA structure lacks a PsaL subunit; instead, a C-terminal domain of IsiA2 occupies the position of PsaL, which inhibits the oligomerization of PSI, leading to the formation of a PSI monomer. Furthermore, excitation-energy transfer from IsiAs to PSI appeared with a time constant of 55 ps. These findings provide insights into both the molecular assembly of the Anabaena IsiA family and the functional roles of IsiAs., (© 2023. The Author(s).)

The cyanobacterium Anabaena sp. strain PCC 7120 exhibits dehydration tolerance. The regulation of gene expression in response to dehydration is crucial for the acquisition of dehydration tolerance, but the molecular mechanisms underlying dehydration responses remain unknown. In this study, the functions of the response regulator OrrA in the regulation of salt and dehydration responses were investigated. Disruption of orrA abolished or diminished the induction of hundreds of genes in response to salt stress and dehydration. Thus, OrrA is a principal regulator of both stress responses. In particular, OrrA plays a crucial role in dehydration tolerance because an orrA disruptant completely lost the ability to regrow after dehydration. Moreover, in the OrrA regulon, avaKa encoding a protein of unknown function was revealed to be indispensable for dehydration tolerance. OrrA and AvaK are conserved among the terrestrial cyanobacteria, suggesting their conserved functions in dehydration tolerance in cyanobacteria., (© 2022 The Authors. Environmental Microbiology published by Society for Applied Microbiology and John Wiley & Sons Ltd.)

Heterocysts are formed in filamentous heterocystous cyanobacteria under nitrogen-starvation conditions, and possess a very low amount of photosystem II (PSII) complexes than vegetative cells. Molecular, morphological, and biochemical characterizations of heterocysts have been investigated; however, excitation-energy dynamics in heterocysts are still unknown. In this study, we examined excitation-energy-relaxation processes of pigment-protein complexes in heterocysts isolated from the cyanobacterium Anabaena sp. PCC 7120. Thylakoid membranes from the heterocysts showed no oxygen-evolving activity under our experimental conditions and no thermoluminescence-glow curve originating from charge recombination of S 2 Q A - . Two dimensional blue-native/SDS-PAGE analysis exhibits tetrameric, dimeric, and monomeric photosystem I (PSI) complexes but almost no dimeric and monomeric PSII complexes in the heterocyst thylakoids. The steady-state fluorescence spectrum of the heterocyst thylakoids at 77 K displays both characteristic PSI fluorescence and unusual PSII fluorescence different from the fluorescence of PSII dimer and monomer complexes. Time-resolved fluorescence spectra at 77 K, followed by fluorescence decay-associated spectra, showed different PSII and PSI fluorescence bands between heterocysts and vegetative thylakoids. Based on these findings, we discuss excitation-energy-transfer mechanisms in the heterocysts., (Copyright © 2021 Elsevier B.V. All rights reserved.)