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Phytochrome photoreceptors control plant growth, development, and the shade avoidance response that limits crop yield in highdensity agricultural plantings. Cyanobacteriochromes (CBCRs) are distantly related photosensory proteins that control cyanobacterial metabolism and behavior in response to light. Photoreceptors in both families reversibly photoconvert between two photostates via photoisomerization of linear tetrapyrrole (bilin) chromophores. Spectroscopic and biochemical studies have demonstrated heterogeneity in both photostates, but the structural basis for such heterogeneity remains unclear. We report solution NMR structures for both photostates of the red/green CBCR NpR6012g4 from Nostoc punctiforme. In addition to identifying structural changes accompanying photoconversion, these structures reveal structural heterogeneity for residues Trp655 and Asp657 in the red-absorbing NpR6012g4 dark state, yielding two distinct environments for the phycocyanobilin chromophore. We use site-directed mutagenesis and fluorescence and absorbance spectroscopy to assign an orangeabsorbing population in the NpR6012g4 dark state to the minority configuration for Asp657. This population does not undergo full, productive photoconversion, as shown by time-resolved spectroscopy and absorption spectroscopy at cryogenic temperature. Our studies thus elucidate the spectral and photochemical consequences of structural heterogeneity in a member of the phytochrome superfamily, insights that should inform efforts to improve photochemical or fluorescence quantum yields in the phytochrome superfamily. [ABSTRACT FROM AUTHOR]

Chloroplasts have evolved from a cyanobacterial endosymbiont and been retained for more than 1 billion years by coordinated chloroplast division in multiplying eukaryotic cells. Chioroplast division is performed by ring structures at the division site, encompassing both the inside and the outside of the two envelopes. A part of the division machinery is derived from the cyanobacterial cytokinetic activity based on the FtsZ protein. In contrast, other parts of the division machinery involve proteins specific to eukaryotes, including a member of the dynamin family. Each member of the dynamin family is involved in the division or fusion of a distinct eukaryotic membrane system. To gain insight into the kind of ancestral dynamin protein and eukaryotic membrane activity that evolved to regulate chloroplast division, we investigated the functions of the dynamin proteins that are most closely related to chloroplast division proteins. These proteins in the amoeba Dictyostelium discoideum and Arabidopsis thaliana localize at the sites of cell division, where they are involved in cytokinesis. Our results suggest that the dynamin for chloroplast division is derived from that involved in eukaryotic cytokinesis. Therefore, the chloroplast division machinery is a mixture of bacterial and eukaryotic cytokinesis components, with the latter a key factor in the synchronization of endosymbiotic cell division with host cell division, thus helping to establish the permanent endosymbiotic relationship. [ABSTRACT FROM AUTHOR]

The composition of photosystem II (PSII) in the chlorophyll (Chl) d-dominated cyanobacterium Acaryochloris marina MBIC 11017 was investigated to enhance the general understanding of the energetics of the PSII reaction center. We first purified photochemically active complexes consisting of a 47-kDa Chl protein (CP47), CP43' (PcbC), D1, D2, cytochrome b559, Psbl, and a small polypeptide. The pigment composition per two pheophytin (Phe) a molecules was 55 ± 7 Chl d, 3.0 ± 0.4 Chl a, 17 ± 3 α-carotene, and 1.4 ± 0.2 plastoquinone-9. The special pair was detected by a reversible absorption change at 713 nm (P713) together with a cation radical band at 842 nm. FTIR difference spectra of the specific bands of a 3-formyl group allowed assignment of the special pair. The combined results indicate that the special pair comprises a Chl d homodimer. The primary electron acceptor was shown by photo-accumulation to be Phe a, and its potential was shifted to a higher value than that in the Chl a/Phe a system. The overall energetics of PSII in the Chl d system are adjusted to changes in the redox potentials, with P713 as the special pair using a lower light energy at 713 nm. Taking into account the reported downward shift in the potential of the special pair of photosystem I (P740) in A. marina, our findings lend support to the idea that changes in photosynthetic pigments combine with a modification of the redox potentials of electron transfer components to give rise to an energetic adjustment of the total reaction system. [ABSTRACT FROM AUTHOR]

In the cyanobacterium Synechococcus elongatus (PCC 7942) the kai genes A, B, and C and the sasA gene encode the functional protein core of the timing mechanism essential for circadian clock regulation of global gene expression. The Kai proteins comprise the central timing mechanism, and the sensor kinase SasA is a primary transducer of temporal information. We demonstrate that the circadian clock also regulates a chromosome compaction rhythm. This chromosome compaction rhythm is both circadian clock-controlled and kai-dependent. Although sasA is required for global gene expression rhythmicity, it is not required for these chromosome compaction rhythms. We also demonstrate direct control by the Kai proteins on the rate at which the SasA protein autophosphorylates. Thus, to generate and maintain circadian rhythms in gene expression, the Kai proteins keep relative time, communicate temporal information to SasA, and may control access to promoter elements by imparting rhythmic chromosome compaction. [ABSTRACT FROM AUTHOR]