Acclimation of the photosynthetic apparatus to low light in a thermophilic Synechococcus sp. strain

Depending upon their growth responses to high and low irradiance, respectively, thermophilic Synechococcus sp. isolates from microbial mats associated with the effluent channels of Mushroom Spring, an alkaline siliceous hot spring in Yellowstone National Park, can be described as either high-light (HL) or low-light (LL) ecotypes. Strains isolated from the bottom of the photic zone grow more rapidly at low irradiance compared to strains isolated from the uppermost layer of the mat, which conversely grow better at high irradiance. The LL-ecotypes develop far-red absorbance and fluorescence emission features after growth in LL. These isolates have a unique gene cluster that encodes a putative cyanobacteriochrome denoted LcyA, a putative sensor histidine kinase; an allophycocyanin (FRL-AP; ApcD4-ApcB3) that absorbs far-red light; and a putative chlorophyll a-binding protein, denoted IsiX, which is homologous to IsiA. The emergence of FRL absorbance in LL-adapted cells of Synechococcus sp. strain A1463 was analyzed in cultures responding to differences in light intensity. The far-red absorbance phenotype arises from expression of a novel antenna complex containing the FRL-AP, ApcD4-ApcB3, which is produced when cells were grown at very low irradiance. Additionally, the two GAF domains of LcyA were shown to bind phycocyanobilin and a [4Fe-4S] cluster, respectively. These ligands potentially enable this photoreceptor to respond to a variety of environmental factors including irradiance, redox potential, and/or oxygen concentration. The products of the gene clusters specific to LL-ecotypes likely facilitate growth in low-light environments through a process called Low-Light Photoacclimation.

The structural basis of far-red light absorbance by allophycocyanins

Phycobilisomes (PBS), the major light-harvesting antenna in cyanobacteria, are supramolecular complexes of colorless linkers and heterodimeric, pigment-binding phycobiliproteins. Phycocyanin and phycoerythrin commonly comprise peripheral rods, and a multi-cylindrical core is principally assembled from allophycocyanin (AP). Each AP subunit binds one phycocyanobilin (PCB) chromophore, a linear tetrapyrrole that predominantly absorbs in the orange-red region of the visible spectrum (600-700 nm). AP facilitates excitation energy transfer from PBS peripheral rods or from directly absorbed red light to accessory chlorophylls in the photosystems. Paralogous forms of AP that bind PCB and are capable of absorbing far-red light (FRL; 700-800 nm) have recently been identified in organisms performing two types of photoacclimation: FRL photoacclimation (FaRLiP) and low-light photoacclimation (LoLiP). The FRL-absorbing AP (FRL-AP) from the thermophilic LoLiP strain Synechococcus sp. A1463 was chosen as a platform for site-specific mutagenesis to probe the structural differences between APs that absorb in the visible region and FRL-APs and to identify residues essential for the FRL absorbance phenotype. Conversely, red light-absorbing allophycocyanin-B (AP-B; ~ 670 nm) from the same organism was used as a platform for creating a FRL-AP. We demonstrate that the protein environment immediately surrounding pyrrole ring A of PCB on the alpha subunit is mostly responsible for the FRL absorbance of FRL-APs. We also show that interactions between PCBs bound to alpha and beta subunits of adjacent protomers in trimeric AP complexes are responsible for a large bathochromic shift of about ~ 20 nm and notable sharpening of the long-wavelength absorbance band.

Caldichromatium japonicum gen. nov., sp. nov., a novel thermophilic phototrophic purple sulphur bacterium of the Chromatiaceae isolated from Nakabusa hot springs, Japan

A novel thermophilic phototrophic purple sulphur bacterium was isolated from microbial mats (56 °C) at Nakabusa hot springs, Nagano prefecture, Japan. Cells were motile, rod-shaped, stain Gram-negative and stored sulphur globules intracellularly. Bacteriochlorophyll a and carotenoids of the normal spirilloxanthin series were the major pigments. Dense liquid cultures were red in colour. Strain No.7^T was able to grow photoautotrophically using sulfide, thiosulfate, sulfite and hydrogen (in the presence of sulfide) as electron donors and bicarbonate as the sole carbon source. Optimum growth occurred under anaerobic conditions in the light at 50 °C (range, 40-56 °C) and pH 7.2 (range, pH 7-8). Major fatty acids were C_16 : 0 (46.8 %), C_16 : 1 ω7c (19.9 %), C_18 : 1 ω7c (21.1 %), C_14 : 0 (4.6 %) and C_18 : 0 (2.4 %). The polar lipid profile showed phosphatidylglycerol and unidentified aminophospholipids to be the major lipids. The only quinone detected was ubiquinone-8. 16S rRNA gene sequence comparisons indicated that the novel bacterium is only distantly related to Thermochromatium tepidum with a nucleotide identity of 90.4 %. The phylogenetic analysis supported the high novelty of strain No.7^T with a long-branching phylogenetic position within the Chromatiaceae next to Thermochromatium tepidum. The genome comprised a circular chromosome of 2.99 Mbp (2 989 870 bp), included no plasmids and had a DNA G+C content of 61.2 mol%. Polyphasic taxonomic analyses of the isolate suggested strain No.7^T is a novel genus within the Chromatiaceae. The proposed genus name of the second truly thermophilic purple sulphur bacterium is Caldichromatium gen. nov. with the type species Caldichromatium japonicum sp. nov. (DSM 110881=JCM 39101).

Characterization of cyanobacterial allophycocyanins absorbing far-red light

Phycobiliproteins (PBPs) are pigment proteins that comprise phycobilisomes (PBS), major light-harvesting antenna complexes of cyanobacteria and red algae. PBS core substructures are made up of allophycocyanins (APs), a subfamily of PBPs. Five paralogous AP subunits are encoded by the Far-Red Light Photoacclimation (FaRLiP) gene cluster, which is transcriptionally activated in cells grown in far-red light (FRL; λ = 700 to 800 nm). FaRLiP gene expression enables some terrestrial cyanobacteria to remodel their PBS and photosystems and perform oxygenic photosynthesis in far-red light (FRL). Paralogous AP genes encoding a putative, FRL-absorbing AP (FRL-AP) are also found in an operon associated with improved low-light growth (LL; < 50 μmol photons m^-2 s^-1) in some thermophilic Synechococcus spp., a phenomenon termed low-light photoacclimation (LoLiP). In this study, apc genes from FaRLiP and LoLiP gene clusters were heterologously expressed individually and in combinations in Escherichia coli. The resulting novel FRL-APs were characterized and identified as major contributors to the FRL absorbance observed in whole cells after FaRLiP and potentially LoLiP. Post-translational modifications of native FRL-APs from FaRLiP cyanobacterium, Leptolyngbya sp. strain JSC-1, were analyzed by mass spectrometry. The PBP complexes made in two FaRLiP organisms were compared, revealing strain-specific diversity in the FaRLiP responses of cyanobacteria. Through analyses of native and recombinant proteins, we improved our understanding of how different cyanobacterial strains utilize specialized APs to acclimate to FRL and LL. We discuss some insights into structural changes that may allow these APs to absorb longer light wavelengths than their visible-light-absorbing paralogs.

Far-red light allophycocyanin subunits play a role in chlorophyll d accumulation in far-red light

Some terrestrial cyanobacteria acclimate to and utilize far-red light (FRL; λ = 700-800 nm) for oxygenic photosynthesis, a process known as far-red light photoacclimation (FaRLiP). A conserved, 20-gene FaRLiP cluster encodes core subunits of Photosystem I (PSI) and Photosystem II (PSII), five phycobiliprotein subunits of FRL-bicylindrical cores, and enzymes for synthesis of chlorophyll (Chl) f and possibly Chl d. Deletion mutants for each of the five apc genes of the FaRLiP cluster were constructed in Synechococcus sp. PCC 7335, and all had similar phenotypes. When the mutants were grown in white (WL) or red (RL) light, the cells closely resembled the wild-type (WT) strain grown under the same conditions. However, the WT and mutant strains were very different when grown under FRL. Mutants grown in FRL were unable to assemble FRL-bicylindrical cores, were essentially devoid of FRL-specific phycobiliproteins, but retained RL-type phycobilisomes and WL-PSII. The transcript levels for genes of the FaRLiP cluster in the mutants were similar to those in WT. Surprisingly, the Chl d contents of the mutant strains were greatly reduced (~ 60-99%) compared to WT and so were the levels of FRL-PSII. We infer that Chl d may be essential for the assembly of FRL-PSII, which does not accumulate to normal levels in the mutants. We further infer that the cysteine-rich subunits of FRL allophycocyanin may either directly participate in the synthesis of Chl d or that FRL bicylindrical cores stabilize FRL-PSII to prevent loss of Chl d.

Cochleovestibular afferent pathways of trapezius muscle responses to clicks in human

Brief intense clicks cause short-latency cervical muscles microcontractions which are supposed to be of vestibular origin. Averaging these microcontractions allows myogenic vestibular evoked potentials (MVEP) to be obtained. MVEP from the trapezius muscles were investigated in normal subjects, cochleovestibular nerve-damaged patients and patients with a vestibular or a cochlear lesion. Muscular responses were recorded on right and left trapezius by averaging from surface electrodes following right and left monaural 100 dB hearing level click stimulation. In normal subjects, responses to monaural stimuli were bilateral, of equal amplitude and latency in left and right trapezia. Normal response consisted of four consecutive waves, labelled p13, n23, p32 and n40 according to their polarity (p, positive; n, negative) and mean peak latency in msec. In total unilateral cochleovestibular damaged patients, auditory stimulation of the affected side gave no MVEP either ipsilateral or contralateral to the stimulation. In the case of selective cochlear lesion, stimulation of the affected side gave MVEP which was present on ipsilateral and contralateral trapezius muscles. The four successive waves were present with a normal latency; however, amplitude was lower than that obtained after stimulation of the healthy ear. In the case of selective vestibular lesion, the four waves of MVEP were again present with normal latency but with reduced amplitude. Responses were present on both the ipsilateral and contralateral trapezius muscle. It is concluded that normal MVEP recorded on the trapezius muscles are bilateral and consist of four waves, the amplitude of which could depend on the simultaneous stimulation of both cochlear and vestibular afferents. In the case of unilateral cochlear and/or vestibular impairments responses were present on both ipsilateral and contralateral trapezius muscles. Latencies had normal values but amplitudes were reduced. MEVP recorded on trapezius muscles were absent in the case of total cochleovestibular damage.