Photocycle of the flavin-binding photoreceptor AppA, a bacterial transcriptional antirepressor of photosynthesis genes

The flavoprotein AppA from Rhodobacter sphaeroides contains an N-terminal domain belonging to a new class of photoreceptors designated BLUF domains. AppA was shown to control photosynthesis gene expression in response to blue light and oxygen tension. We have investigated the photocycle of the AppA BLUF domain by ultrafast fluorescence, femtosecond transient absorption, and nanosecond flash-photolysis spectroscopy. Time-resolved fluorescence experiments revealed four components of flavin adenine dinucleotide (FAD) excited-state decay, with lifetimes of 25 ps, 150 ps, 670 ps, and 3.8 ns. Ultrafast transient absorption spectroscopy revealed rapid internal conversion and vibrational cooling processes on excited FAD with time constants of 250 fs and 1.2 ps, and a multiexponential decay with effective time constants of 90 ps, 590 ps, and 2.7 ns. Concomitant with the decay of excited FAD, the rise of a species with a narrow absorption difference band near 495 nm was detected which spectrally resembles the long-living signaling state of AppA. Consistent with these results, the nanosecond flash-photolysis measurements indicated that formation of the signaling state was complete within the time resolution of 10 ns. No further changes were detected up to 15 micros. The quantum yield of the signaling-state formation was determined to be 24%. Thus, the signaling state of the AppA BLUF domain is formed on the ultrafast time scale directly from the FAD singlet excited state, without any apparent intermediate, and remains stable over 12 decades of time. In parallel with the signaling state, the FAD triplet state is formed from the FAD singlet excited state at 9% efficiency as a side reaction of the AppA photocycle.

Comparison of dopamine and noradrenaline release in mouse prefrontal cortex, striatum and hippocampus using microdialysis

In vivo release of dopamine (DA) and noradrenaline (NA) in mouse medial prefrontal cortex, medial striatum and hippocampus was characterized using in vivo microdialysis. Basal release of NA was similar in these areas, but DA in striatum was 13-30 times higher than in other areas. Unconditioned stimuli (handling, novelty) induced strong increases, except for striatal DA. Striatal NA was more sensitive to handling than NA in other areas.

Supramolecular organization and dual function of the IsiA chlorophyll-binding protein in cyanobacteria

A significant part of global primary productivity is provided by cyanobacteria, which are abundant in most marine and freshwater habitats. In many oceanographic regions, however, the concentration of iron can be so low that it limits growth. Cyanobacteria respond to this condition by expressing a number of iron stress inducible genes, of which the isiA gene encodes a chlorophyll-binding protein known as IsiA or CP43'. It was recently shown that 18 IsiA proteins encircle trimeric photosystem I (PSI) under iron-deficient growth conditions. We report here that after prolonged growth of Synechocystis PCC 6803 in an iron-deficient medium, the number of bound IsiA proteins can be much higher than previously known. The largest complexes bind 12-14 units in an inner ring and 19-21 units in an outer ring around a PSI monomer. Fluorescence excitation spectra indicate an efficient light harvesting function for all PSI-bound chlorophylls. We also find that IsiA accumulates in cyanobacteria in excess of what is needed for functional light harvesting by PSI, and that a significant part of IsiA builds supercomplexes without PSI. Because the further decline of PSI makes photosystem II (PSII) increasingly vulnerable to photooxidation, we postulate that the surplus synthesis of IsiA shields PSII from excess light. We suggest that IsiA plays a surprisingly versatile role in cyanobacteria, by significantly enhancing the light harvesting ability of PSI and providing photoprotection for PSII.

Unraveling the interaction between doxorubicin and DNA origami nanostructures for customizable chemotherapeutic drug release

Doxorubicin (DOX) is a common drug in cancer chemotherapy, and its high DNA-binding affinity can be harnessed in preparing DOX-loaded DNA nanostructures for targeted delivery and therapeutics. Although DOX has been widely studied, the existing literature of DOX-loaded DNA-carriers remains limited and incoherent. Here, based on an in-depth spectroscopic analysis, we characterize and optimize the DOX loading into different 2D and 3D scaffolded DNA origami nanostructures (DONs). In our experimental conditions, all DONs show similar DOX binding capacities (one DOX molecule per two to three base pairs), and the binding equilibrium is reached within seconds, remarkably faster than previously acknowledged. To characterize drug release profiles, DON degradation and DOX release from the complexes upon DNase I digestion was studied. For the employed DONs, the relative doses (DOX molecules released per unit time) may vary by two orders of magnitude depending on the DON superstructure. In addition, we identify DOX aggregation mechanisms and spectral changes linked to pH, magnesium, and DOX concentration. These features have been largely ignored in experimenting with DNA nanostructures, but are probably the major sources of the incoherence of the experimental results so far. Therefore, we believe this work can act as a guide to tailoring the release profiles and developing better drug delivery systems based on DNA-carriers.

Structural photoactivation of a full-length bacterial phytochrome

Phytochromes are light sensor proteins found in plants, bacteria, and fungi. They function by converting a photon absorption event into a conformational signal that propagates from the chromophore through the entire protein. However, the structure of the photoactivated state and the conformational changes that lead to it are not known. We report time-resolved x-ray scattering of the full-length phytochrome from Deinococcus radiodurans on micro- and millisecond time scales. We identify a twist of the histidine kinase output domains with respect to the chromophore-binding domains as the dominant change between the photoactivated and resting states. The time-resolved data further show that the structural changes up to the microsecond time scales are small and localized in the chromophore-binding domains. The global structural change occurs within a few milliseconds, coinciding with the formation of the spectroscopic meta-Rc state. Our findings establish key elements of the signaling mechanism of full-length bacterial phytochromes.

Folding and unfolding of a photoswitchable peptide from picoseconds to microseconds

Using time-resolved IR spectroscopy, we monitored the kinetics of folding and unfolding processes of a photoswitchable 16-residue alanine-based alpha-helical peptide on a timescale from few picoseconds to almost 40 micros and over a large temperature range (279-318 K). The folding and unfolding processes were triggered by an ultrafast laser pulse that isomerized the cross linker within a few picoseconds. The main folding and unfolding times (700 ns and 150 ns, respectively, at room temperature) are in line with previous T-jump experiments obtained from similar peptides. However, both processes show complex, strongly temperature-dependent spectral kinetics that deviate clearly from a single-exponential behavior. Whereas in the unfolding experiment the ensemble starts from a well defined folded state, the starting ensemble in the folding experiment is more heterogeneous, which leads to distinctly different kinetics of the experiments, because they are sensitive to different regions of the energy surface. A qualitative agreement with the experimental data-set can be obtained by a model where the unfolded states act as a hub connected to several separated "misfolded" states with a distribution of rates. We conclude that a rather large spread of rates (k(1) : k(n) approximately 9) is needed to explain the experimentally observed stretched exponential response with stretching factor beta = 0.8 at 279 K.

The primary structural photoresponse of phytochrome proteins captured by a femtosecond X-ray laser

Phytochrome proteins control the growth, reproduction, and photosynthesis of plants, fungi, and bacteria. Light is detected by a bilin cofactor, but it remains elusive how this leads to activation of the protein through structural changes. We present serial femtosecond X-ray crystallographic data of the chromophore-binding domains of a bacterial phytochrome at delay times of 1 ps and 10 ps after photoexcitation. The data reveal a twist of the D-ring, which leads to partial detachment of the chromophore from the protein. Unexpectedly, the conserved so-called pyrrole water is photodissociated from the chromophore, concomitant with movement of the A-ring and a key signaling aspartate. The changes are wired together by ultrafast backbone and water movements around the chromophore, channeling them into signal transduction towards the output domains. We suggest that the observed collective changes are important for the phytochrome photoresponse, explaining the earliest steps of how plants, fungi and bacteria sense red light.

Plants adapt to the availability of light throughout their lives because it regulates so many aspects of their growth and reproduction. To detect the level of light, plant cells use proteins called phytochromes, which are also found in some bacteria and fungi. Phytochrome proteins change shape when they are exposed to red light, and this change alters the behaviour of the cell. The red light is absorbed by a molecule known as chromophore, which is connected to a region of the phytochrome called the PHY-tongue. This region undergoes one of the key structural changes that occur when the phytochrome protein absorbs light, turning from a flat sheet into a helix.

Claesson, Wahlgren, Takala et al. studied the structure of a bacterial phytochrome protein almost immediately after shining a very brief flash of red light using a laser. The experiments revealed that the structure of the protein begins to change within a trillionth of a second: specifically, the chromophore twists, which disrupts its attachment to the protein, freeing the protein to change shape. Claesson, Wahlgren, Takala et al. note that this structure is likely a very short-lived intermediate state, which however triggers more changes in the overall shape change of the protein.

One feature of the rearrangement is the disappearance of a particular water molecule. This molecule can be found at the core of many different phytochrome structures and interacts with several parts of the chromophore and the phytochrome protein. It is unclear why the water molecule is lost, but given how quickly this happens after the red light is applied it is likely that this disappearance is an integral part of the reshaping process.

Together these events disrupt the interactions between the chromophore and the PHY-tongue, enabling the PHY-tongue to change shape and alter the structure of the phytochrome protein. Understanding and controlling this process could allow scientists to alter growth patterns in plants, such as crops or weeds.

Aggregates of the chlorophyll-binding protein IsiA (CP43') dissipate energy in cyanobacteria

In many natural habitats, growth of cyanobacteria may be limited by a low concentration of iron. Cyanobacteria respond to this condition by expressing a number of iron-stress-inducible genes, of which the isiA gene encodes a chlorophyll-binding protein known as IsiA or CP43'. IsiA monomers assemble into ring-shaped polymers that encircle trimeric or monomeric photosystem I (PSI), or are present in supercomplexes without PSI, in particular upon prolonged iron starvation. In this report, we present steady-state and time-resolved fluorescence measurements of isolated IsiA aggregates that have been purified from an iron-starved psaFJ-minus mutant of Synechocystis PCC 6803. We show that these aggregates have a fluorescence quantum yield of approximately 2% compared to that of chlorophyll a in acetone, and that the dominating fluorescence lifetimes are 66 and 210 ps, more than 1 order of magnitude shorter than that of free chlorophyll a. Comparison of the temperature dependence of the fluorescence yields and spectra of the isolated aggregates and of the cells from which they were obtained suggests that these aggregates occur naturally in the iron-starved cells. We suggest that IsiA aggregates protect cyanobacterial cells against the deleterious effects of light.

Pigment organization and energy transfer dynamics in isolated photosystem I (PSI) complexes from Arabidopsis thaliana depleted of the PSI-G, PSI-K, PSI-L, or PSI-N subunit

Green plant photosystem I (PSI) consists of at least 18 different protein subunits. The roles of some of these protein subunits are not well known, in particular those that do not occur in the well characterized PSI complexes from cyanobacteria. We investigated the spectroscopic properties and excited-state dynamics of isolated PSI-200 particles from wild-type and mutant Arabidopsis thaliana plants devoid of the PSI-G, PSI-K, PSI-L, or PSI-N subunit. Pigment analysis and a comparison of the 5 K absorption spectra of the various particles suggests that the PSI-L and PSI-H subunits together bind approximately five chlorophyll a molecules with absorption maxima near 688 and 667 nm, that the PSI-G subunit binds approximately two red-shifted beta-carotene molecules, that PSI-200 particles without PSI-K lack a part of the peripheral antenna, and that the PSI-N subunit does not bind pigments. Measurements of fluorescence decay kinetics at room temperature with picosecond time resolution revealed lifetimes of ~0.6, 5, 15, 50, 120, and 5000 ps in all particles. The 5- and 15-ps phases could, at least in part, be attributed to the excitation equilibration between bulk and red chlorophyll forms, though the 15-ps phase also contains a contribution from trapping by charge separation. The 50- and 120-ps phases predominantly reflect trapping by charge separation. We suggest that contributions from the core antenna dominate the 15-ps trapping phase, that those from the peripheral antenna proteins Lhca2 and Lhca3 dominate the 50-ps phase, and that those from Lhca1 and Lhca4 dominate the 120-ps phase. In the PSI-200 particles without PSI-K or PSI-G protein, more excitations are trapped in the 15-ps phase and less in 50- and 120-ps phases, which is in agreement with the notion that these subunits are involved in the interaction between the core and peripheral antenna proteins.

Bioactive glass ions as strong enhancers of osteogenic differentiation in human adipose stem cells

Bioactive glasses are known for their ability to induce osteogenic differentiation of stem cells. To elucidate the mechanism of the osteoinductivity in more detail, we studied whether ionic extracts prepared from a commercial glass S53P4 and from three experimental glasses (2-06, 1-06 and 3-06) are alone sufficient to induce osteogenic differentiation of human adipose stem cells. Cells were cultured using basic medium or osteogenic medium as extract basis. Our results indicate that cells stay viable in all the glass extracts for the whole culturing period, 14 days. At 14 days the mineralization in osteogenic medium extracts was excessive compared to the control. Parallel to the increased mineralization we observed a decrease in the cell amount. Raman and Laser Induced Breakdown Spectroscopy analyses confirmed that the mineral consisted of calcium phosphates. Consistently, the osteogenic medium extracts also increased osteocalcin production and collagen Type-I accumulation in the extracellular matrix at 13 days. Of the four osteogenic medium extracts, 2-06 and 3-06 induced the best responses of osteogenesis. However, regardless of the enhanced mineral formation, alkaline phosphatase activity was not promoted by the extracts. The osteogenic medium extracts could potentially provide a fast and effective way to differentiate human adipose stem cells in vitro.

Kinetics of excitation trapping in intact Photosystem I of Chlamydomonas reinhardtii and Arabidopsis thaliana

We measured picosecond time-resolved fluorescence of intact Photosystem I complexes from Chlamydomonas reinhardtii and Arabidopsis thaliana. The antenna system of C. reinhardtii contains about 30-60 chlorophylls more than that of A. thaliana, but lacks the so-called red chlorophylls, chlorophylls that absorb at longer wavelength than the primary electron donor. In C. reinhardtii, the main lifetimes of excitation trapping are about 27 and 68 ps. The overall lifetime of C. reinhardtii is considerably shorter than in A. thaliana. We conclude that the amount and energies of the red chlorophylls have a larger effect on excitation trapping time in Photosystem I than the antenna size.

Structure of the Higher Plant Light Harvesting Complex I: In Vivo Characterization and Structural Interdependence of the Lhca Proteins

We have investigated the structure of the higher plant light harvesting complex of photosystem I (LHCI) by analyzing PSI-LHCI particles isolated from a set of Arabidopsis plant lines, each lacking a specific Lhca (Lhca1-4) polypeptide. Functional antenna size measurements support the recent finding that there are four Lhca proteins per PSI in the crystal structure [Ben-Shem, A., Frolow, F., and Nelson, N. (2003) Nature 426, 630-635]. According to HPLC analyses the number of pigment molecules bound within the LHCI is higher than expected from reconstitution studies or analyses of isolated native LHCI. Comparison of the spectra of the particles from the different lines reveals chlorophyll absorption bands peaking at 696, 688, 665, and 655 nm that are not present in isolated PSI or LHCI. These bands presumably originate from "gap" or "linker" pigments that are cooperatively coordinated by the Lhca and/or PSI proteins, which we have tentatively localized in the PSI-LHCI complex.

Origins of fluorescence in evolved bacteriophytochromes

Use of fluorescent proteins to study in vivo processes in mammals requires near-infrared (NIR) biomarkers that exploit the ability of light in this range to penetrate tissue. Bacteriophytochromes (BphPs) are photoreceptors that couple absorbance of NIR light to photoisomerization, protein conformational changes, and signal transduction. BphPs have been engineered to form NIR fluorophores, including IFP1.4, Wi-Phy, and the iRFP series, initially by replacement of Asp-207 by His. This position was suggestive because its main chain carbonyl is within hydrogen-bonding distance to pyrrole ring nitrogens of the biliverdin chromophore, thus potentially functioning as a crucial transient proton sink during photoconversion. To explain the origin of fluorescence in these phytofluors, we solved the crystal structures of IFP1.4 and a comparison non-fluorescent monomeric phytochrome DrCBDmon. Met-186 and Val-288 in IFP1.4 are responsible for the formation of a tightly packed hydrophobic hub around the biliverdin D ring. Met-186 is also largely responsible for the blue-shifted IFP1.4 excitation maximum relative to the parent BphP. The structure of IFP1.4 revealed decreased structural heterogeneity and a contraction of two surface regions as direct consequences of side chain substitutions. Unexpectedly, IFP1.4 with Asp-207 reinstalled (IFPrev) has a higher fluorescence quantum yield (∼9%) than most NIR phytofluors published to date. In agreement, fluorescence lifetime measurements confirm the exceptionally long excited state lifetimes, up to 815 ps, in IFP1.4 and IFPrev. Our research helps delineate the origin of fluorescence in engineered BphPs and will facilitate the wide-spread adoption of phytofluors as biomarkers.

The low-energy forms of photosystem I light-harvesting complexes: spectroscopic properties and pigment-pigment interaction characteristics

In this work the spectroscopic properties of the special low-energy absorption bands of the outer antenna complexes of higher plant Photosystem I have been investigated by means of low-temperature absorption, fluorescence, and fluorescence line-narrowing experiments. It was found that the red-most absorption bands of Lhca3, Lhca4, and Lhca1-4 peak, respectively, at 704, 708, and 709 nm and are responsible for 725-, 733-, and 732-nm fluorescence emission bands. These bands are more red shifted compared to "normal" chlorophyll a (Chl a) bands present in light-harvesting complexes. The low-energy forms are characterized by a very large bandwidth (400-450 cm(-1)), which is the result of both large homogeneous and inhomogeneous broadening. The observed optical reorganization energy is untypical for Chl a and resembles more that of BChl a antenna systems. The large broadening and the changes in optical reorganization energy are explained by a mixing of an Lhca excitonic state with a charge transfer state. Such a charge transfer state can be stabilized by the polar residues around Chl 1025. It is shown that the optical reorganization energy is changing through the inhomogeneous distribution of the red-most absorption band, with the pigments contributing to the red part of the distribution showing higher values. A second red emission form in Lhca4 was detected at 705 nm and originates from a broad absorption band peaking at 690 nm. This fluorescence emission is present also in the Lhca4-N-47H mutant, which lacks the 733-nm emission band.

Evidence for two spectroscopically different dimers of light-harvesting complex I from green plants

A preparation consisting of isolated dimeric peripheral antenna complexes from green plant photosystem I (light-harvesting complex I or LHCI) has been characterized by means of (polarized) steady-state absorption and fluorescence spectroscopy at low temperatures. We show that this preparation can be described reasonably well by a mixture of two types of dimers. In the first dimer about 10% of all Q(y)() absorption of the chlorophylls arises from two chlorophylls with absorption and emission maxima at about 711 and 733 nm, respectively, whereas in the second about 10% of the absorption arises from two chlorophylls with absorption and emission maxima at about 693 and 702 nm, respectively. The remaining chlorophylls show spectroscopic properties comparable to those of the related peripheral antenna complexes of photosystem II. We attribute the first dimer to a heterodimer of the Lhca1 and Lhca4 proteins and the second to a hetero- or homodimer of the Lhca2 and/or Lhca3 proteins. We suggest that the chlorophylls responsible for the 733 nm emission (F-730) and 702 nm emission (F-702) are excitonically coupled dimers and that F-730 originates from one of the strongest coupled pair of chlorophylls observed in nature.

On the (un)coupling of the chromophore, tongue interactions, and overall conformation in a bacterial phytochrome

Phytochromes are photoreceptors in plants, fungi, and various microorganisms and cycle between metastable red light-absorbing (Pr) and far-red light-absorbing (Pfr) states. Their light responses are thought to follow a conserved structural mechanism that is triggered by isomerization of the chromophore. Downstream structural changes involve refolding of the so-called tongue extension of the phytochrome-specific GAF-related (PHY) domain of the photoreceptor. The tongue is connected to the chromophore by conserved DIP and PRXSF motifs and a conserved tyrosine, but the role of these residues in signal transduction is not clear. Here, we examine the tongue interactions and their interplay with the chromophore by substituting the conserved tyrosine (Tyr²⁶³) in the phytochrome from the extremophile bacterium Deinococcus radiodurans with phenylalanine. Using optical and FTIR spectroscopy, X-ray solution scattering, and crystallography of chromophore-binding domain (CBD) and CBD-PHY fragments, we show that the absence of the Tyr²⁶³ hydroxyl destabilizes the β-sheet conformation of the tongue. This allowed the phytochrome to adopt an α-helical tongue conformation regardless of the chromophore state, hence distorting the activity state of the protein. Our crystal structures further revealed that water interactions are missing in the Y263F mutant, correlating with a decrease of the photoconversion yield and underpinning the functional role of Tyr²⁶³ in phytochrome conformational changes. We propose a model in which isomerization of the chromophore, refolding of the tongue, and globular conformational changes are represented as weakly coupled equilibria. The results also suggest that the phytochromes have several redundant signaling routes.

Light-induced Changes in the Dimerization Interface of Bacteriophytochromes

Phytochromes are dimeric photoreceptor proteins that sense red light levels in plants, fungi, and bacteria. The proteins are structurally divided into a light-sensing photosensory module consisting of PAS, GAF, and PHY domains and a signaling output module, which in bacteriophytochromes typically is a histidine kinase (HK) domain. Existing structural data suggest that two dimerization interfaces exist between the GAF and HK domains, but their functional roles remain unclear. Using mutational, biochemical, and computational analyses of the Deinococcus radiodurans phytochrome, we demonstrate that two dimerization interfaces between sister GAF and HK domains stabilize the dimer with approximately equal contributions. The existence of both dimerization interfaces is critical for thermal reversion back to the resting state. We also find that a mutant in which the interactions between the GAF domains were removed monomerizes under red light. This implies that the interactions between the HK domains are significantly altered by photoconversion. The results suggest functional importance of the dimerization interfaces in bacteriophytochromes.

Excitation energy transfer in native and unstacked thylakoid membranes studied by low temperature and ultrafast fluorescence spectroscopy

In this work, the transfer of excitation energy was studied in native and cation-depletion induced, unstacked thylakoid membranes of spinach by steady-state and time-resolved fluorescence spectroscopy. Fluorescence emission spectra at 5 K show an increase in photosystem I (PSI) emission upon unstacking, which suggests an increase of its antenna size. Fluorescence excitation measurements at 77 K indicate that the increase of PSI emission upon unstacking is caused both by a direct spillover from the photosystem II (PSII) core antenna and by a functional association of light-harvesting complex II (LHCII) to PSI, which is most likely caused by the formation of LHCII-LHCI-PSI supercomplexes. Time-resolved fluorescence measurements, both at room temperature and at 77 K, reveal differences in the fluorescence decay kinetics of stacked and unstacked membranes. Energy transfer between LHCII and PSI is observed to take place within 25 ps at room temperature and within 38 ps at 77 K, consistent with the formation of LHCII-LHCI-PSI supercomplexes. At the 150-160 ps timescale, both energy transfer from LHCII to PSI as well as spillover from the core antenna of PSII to PSI is shown to occur at 77 K. At room temperature the spillover and energy transfer to PSI is less clear at the 150 ps timescale, because these processes compete with charge separation in the PSII reaction center, which also takes place at a timescale of about 150 ps.

2D-IR study of a photoswitchable isotope-labeled alpha-helix

A series of photoswitchable, alpha-helical peptides were studied using two-dimensional infrared spectroscopy (2D-IR). Single-isotope labeling with (13)C(18)O at various positions in the sequence was employed to spectrally isolate particular backbone positions. We show that a single (13)C(18)O label can give rise to two bands along the diagonal of the 2D-IR spectrum, one of which is from an amide group that is hydrogen-bonded internally, or to a solvent molecule, and the other from a non-hydrogen-bonded amide group. The photoswitch enabled examination of both the folded and unfolded state of the helix. For most sites, unfolding of the peptide caused a shift of intensity from the hydrogen-bonded peak to the non-hydrogen-bonded peak. The relative intensity of the two diagonal peaks gives an indication of the fraction of molecules hydrogen-bonded at a certain location along the sequence. As this fraction varies quite substantially along the helix, we conclude that the helix is not uniformly folded. Furthermore, the shift in hydrogen bonding is much smaller than the change of helicity measured by CD spectroscopy, indicating that non-native hydrogen-bonded or mis-folded loops are formed in the unfolded ensemble.

Origin of the 701-nm Fluorescence Emission of the Lhca2 Subunit of Higher Plant Photosystem I

Photosystem I of higher plants is characterized by red-shifted spectral forms deriving from chlorophyll chromophores. Each of the four Lhca1 to -4 subunits exhibits a specific fluorescence emission spectrum, peaking at 688, 701, 725, and 733 nm, respectively. Recent analysis revealed the role of chlorophyll-chlorophyll interactions of the red forms in Lhca3 and Lhca4, whereas the basis for the fluorescence emission at 701 nm in Lhca2 is not yet clear. We report a detailed characterization of the Lhca2 subunit using molecular biology, biochemistry, and spectroscopy and show that the 701-nm emission form originates from a broad absorption band at 690 nm. Spectroscopy on recombinant mutant proteins assesses that this band represents the low energy form of an excitonic interaction involving two chlorophyll a molecules bound to sites A5 and B5, the same protein domains previously identified for Lhca3 and Lhca4. The resulting emission is, however, substantially shifted to higher energies. These results are discussed on the basis of the structural information that recently became available from x-ray crystallography (Ben Shem, A., Frolow, F., and Nelson, N. (2003) Nature 426, 630-635). We suggest that, within the Lhca subfamily, spectroscopic properties of chromophores are modulated by the strength of the excitonic coupling between the chromophores A5 and B5, thus yielding fluorescence emission spanning a large wavelength interval. It is concluded that the interchromophore distance rather than the transition energy of the individual chromophores or the orientation of transition vectors represents the critical factor in determining the excitonic coupling in Lhca pigment-protein complexes.