Role of apparent membrane growth initiation sites during photosynthetic membrane development in synchronously dividing Rhodopseudomonas sphaeroides

Membrane development in purple photosynthetic bacteria in response to alterations in light intensity and oxygen tension

Studies on membrane development in purple bacteria during adaptation to alterations in light intensity and oxygen tension are reviewed. Anoxygenic phototrophic such as the purple α-proteobacterium Rhodobacter sphaeroides have served as simple, dynamic, and experimentally accessible model organisms for studies of the photosynthetic apparatus. A major landmark in photosynthesis research, which dramatically illustrates this point, was provided by the determination of the X-ray structure of the reaction center (RC) in Blastochloris viridis (Deisenhofer and Michel, EMBO J 8:2149-2170, 1989), once it was realized that this represented the general structure for the photosystem II RC present in all oxygenic phototrophs. This seminal advance, together with a considerable body of subsequent research on the light-harvesting (LH) and electron transfer components of the photosynthetic apparatus has provided a firm basis for the current understanding of how phototrophs acclimate to alterations in light intensity and quality. Oxygenic phototrophs adapt to these changes by extensive thylakoid membrane remodeling, which results in a dramatic supramolecular reordering to assure that an appropriate flow of quinone redox species occurs within the membrane bilayer for efficient and rapid electron transfer. Despite the high level of photosynthetic unit organization in Rba. sphaeroides as observed by atomic force microscopy (AFM), fluorescence induction/relaxation measurements have demonstrated that the addition of the peripheral LH2 antenna complex in cells adapting to low-intensity illumination results in a slowing of the rate of electron transfer turnover by the RC of up to an order of magnitude. This is ascribed to constraints in quinone redox species diffusion between the RC and cytochrome bc1 complexes arising from the increased packing density as the intracytoplasmic membrane (ICM) bilayer becomes crowded with LH2 rings. In addition to downshifts in light intensity as a paradigm for membrane development studies in Rba. sphaeroides, the lowering of oxygen tension in chemoheterotropically growing cells results in a gratuitous formation of the ICM by an extensive membrane biogenesis process. These membrane alterations in response to lowered illumination and oxygen levels in purple bacteria are under the control of a number of interrelated two-component regulatory circuits reviewed here, which act at the transcriptional level to regulate the formation of both the pigment and apoprotein components of the LH, RC, and respiratory complexes. We have performed a proteomic examination of the ICM development process in which membrane proteins have been identified that are temporally expressed both during adaptation to low light intensity and ICM formation at low aeration and are spatially localized in both growing and mature ICM regions. For these proteomic analyses, membrane growth initiation sites and mature ICM vesicles were isolated as respective upper-pigmented band (UPB) and chromatophore fractions and subjected to clear native electrophoresis for isolation of bands containing the LH2 and RC-LH1 core complexes. In chromatophores, increasing levels of LH2 polypeptides relative to those of the RC-LH1 complex were observed as ICM membrane development proceeded during light-intensity downshifts, along with a large array of other associated proteins including high spectral counts for the F1FO-ATP synthase subunits and the cytochrome bc1 complex, as well as RSP6124, a protein of unknown function, that was correlated with increasing LH2 spectral counts. In contrast, the UPB was enriched in cytoplasmic membrane (CM) markers, including electron transfer and transport proteins, as well as general membrane protein assembly factors confirming the origin of the UPB from both peripheral respiratory membrane and sites of active CM invagination that give rise to the ICM. The changes in ICM vesicles were correlated to AFM mapping results (Adams and Hunter, Biochim Biophys Acta 1817:1616-1627, 2012), in which the increasing LH2 levels were shown to form densely packed LH2-only domains, representing the light-responsive antenna complement formed under low illumination. The advances described here could never have been envisioned when the author was first introduced in the mid-1960s to the intricacies of the photosynthetic apparatus during a lecture delivered in a graduate Biochemistry course at the University of Illinois by Govindjee, to whom this volume is dedicated on the occasion of his 80th birthday.

Adaptation of intracytoplasmic membranes to altered light intensity in Rhodobacter sphaeroides

The model photosynthetic bacterium Rhodobacter sphaeroides uses a network of bacteriochlorophyll (BChl)-protein complexes embedded in spherical intracytoplasmic membranes (ICM) to collect and utilise solar energy. We studied the effects of high- and low-light growth conditions, where BChl levels increased approximately four-fold from 1.6×10(6) to 6.5×10(6) molecules per cell. Most of this extra pigment is accommodated in the proliferating ICM system, which increases from approximately 274 to 1468 vesicles per cell. Thus, 16×10(6)nm(2) of specialised membrane surface area is made available for harvesting and utilising solar energy compared to 3×10(6)nm(2) under high-light conditions. Membrane mapping using atomic force microscopy revealed closely packed dimeric and monomeric reaction centre-light harvesting 1-PufX (RC-LH1-PufX) complexes in high-light ICM with room only for small clusters of LH2, whereas extensive LH2-only domains form during adaptation to low light, with the LH2/RC ratio increasing three-fold. The number of upper pigmented band (UPB) sites where membrane invagination is initiated hardly varied; 704 (5.8×10(5) BChls/cell) and 829 (4.9×10(5) BChls/cell) UPB sites per cell were estimated under high- and low-light conditions, respectively. Thus, the lower ICM content in high-light cells is a consequence of fewer ICM invaginations reaching maturity. Taking into account the relatively poor LH2-to-LH1 energy transfer in UPB membranes it is likely that high-light cells are relatively inefficient at energy trapping, but can grow well enough without the need to fully develop their photosynthetic membranes from the relatively inefficient UPB to highly efficient mature ICM.

Quantitative proteomic analysis of intracytoplasmic membrane development in Rhodobacter sphaeroides

The purple phototrophic bacteria elaborate a specialized intracytoplasmic membrane (ICM) system for the conversion of solar energy to ATP. Previous radiolabelling and ultrastructural experiments have shown that ICM assembly in Rhodobacter sphaeroides is initiated at indentations of the cytoplasmic membrane, termed UPB. Here, we report proteomic analyses of precursor (UPB) and mature (ICM) fractions. Qualitative data identified 387 proteins, only 43 of which were found in the ICM, reflecting its specialized role within the cell, the conversion of light into chemical energy; 236 proteins were found in the significantly more complex UPB proteome. Metabolic labelling was used to quantify the relative distribution of 173 proteins between the UPB and ICM fractions. Quantification reveals new information on assembly of the RC-LH1-PufX, ATP synthase and NAD(P)H transhydrogenase complexes, as well as showing that the UPB is enriched in enzymes for lipid, carbohydrate and amino acid metabolism, tetrapyrrole biosynthesis and proteins representing a wide range of other metabolic and biosynthetic functions. Proteins involved in light harvesting, photochemistry, electron transport and ATP synthesis are all enriched in ICM, consistent with the spatial proximity of energy capturing and transducing functions. These data provide further support to the developmental precursor-product relationship between UPB and ICM.

Monomeric RC-LH1 core complexes retard LH2 assembly and intracytoplasmic membrane formation in PufX-minus mutants of Rhodobacter sphaeroides

In the model photosynthetic bacterium Rhodobacter sphaeroides domains of light-harvesting 2 (LH2) complexes surround and interconnect dimeric reaction centre-light-harvesting 1-PufX (RC-LH1-PufX) 'core' complexes, forming extensive networks for energy transfer and trapping. These complexes are housed in spherical intracytoplasmic membranes (ICMs), which are assembled in a stepwise process where biosynthesis of core complexes tends to dominate the early stages of membrane invagination. The kinetics of LH2 assembly were measured in PufX mutants that assemble monomeric core complexes, as a consequence of either a twelve-residue N-terminal truncation of PufX (PufXΔ12) or the complete removal of PufX (PufX(-)). Lower rates of LH2 assembly and retarded maturation of membrane invagination were observed for the larger and less curved ICM from the PufX(-) mutant, consistent with the proposition that local membrane curvature, initiated by arrays of bent RC-LH1-PufX dimers, creates a favourable environment for stable assembly of LH2 complexes. Transmission electron microscopy and high-resolution atomic force microscopy were used to examine ICM morphology and membrane protein organisation in these mutants. Some partitioning of core and LH2 complexes was observed in PufX(-) membranes, resulting in locally ordered clusters of monomeric RC-LH1 complexes. The distribution of core and LH2 complexes in the three types of membrane examined is consistent with previous models of membrane curvature and domain formation (Frese et al., 2008), which demonstrated that a combination of crowding and asymmetries in sizes and shapes of membrane protein complexes drives membrane organisation.

Membrane invagination in Rhodobacter sphaeroides is initiated at curved regions of the cytoplasmic membrane, then forms both budded and fully detached spherical vesicles

The purple phototrophic bacteria synthesize an extensive system of intracytoplasmic membranes (ICM) in order to increase the surface area for absorbing and utilizing solar energy. Rhodobacter sphaeroides cells contain curved membrane invaginations. In order to study the biogenesis of ICM in this bacterium mature (ICM) and precursor (upper pigmented band - UPB) membranes were purified and compared at the single membrane level using electron, atomic force and fluorescence microscopy, revealing fundamental differences in their morphology, protein organization and function. Cryo-electron tomography demonstrates the complexity of the ICM of Rba. sphaeroides. Some ICM vesicles have no connection with other structures, others are found nearer to the cytoplasmic membrane (CM), often forming interconnected structures that retain a connection to the CM, and possibly having access to the periplasmic space. Near-spherical single invaginations are also observed, still attached to the CM by a 'neck'. Small indents of the CM are also seen, which are proposed to give rise to the UPB precursor membranes upon cell disruption. 'Free-living' ICM vesicles, which possess all the machinery for converting light energy into ATP, can be regarded as bacterial membrane organelles.

Sequential assembly of photosynthetic units in Rhodobacter sphaeroides as revealed by fast repetition rate analysis of variable bacteriochlorophyll a fluorescence

The development of functional photosynthetic units in Rhodobacter sphaeroides was followed by near infra-red fast repetition rate (IRFRR) fluorescence measurements that were correlated to absorption spectroscopy, electron microscopy and pigment analyses. To induce the formation of intracytoplasmic membranes (ICM) (greening), cells grown aerobically both in batch culture and in a carbon-limited chemostat were transferred to semiaerobic conditions. In both aerobic cultures, a low level of photosynthetic complexes was observed, which were composed of the reaction center and the LH1 core antenna. Interestingly, in the batch cultures the reaction centers were essentially inactive in forward electron transfer and exhibited low photochemical yields F(V)/F(M), whereas the chemostat culture displayed functional reaction centers with a rather rapid (1-2 ms) electron transfer turnover, as well as a high F(V)/F(M) of approximately 0.8. In both cases, the transfer to semiaerobiosis resulted in rapid induction of bacteriochlorophyll a synthesis that was reflected by both an increase in the number of LH1-reaction center and peripheral LH2 antenna complexes. These studies establish that photosynthetic units are assembled in a sequential manner, where the appearance of the LH1-reaction center cores is followed by the activation of functional electron transfer, and finally by the accumulation of the LH2 complexes.

The assembly and organisation of photosynthetic membranes in Rhodobacter sphaeroides

Recent AFM data demonstrate that mature photosynthetic membranes of R. sphaeroides are composed of rows of dimeric RC-LH1-PufX complexes with some LH2 complexes 'sandwiched' between these rows of core complexes, and others in discrete LH2-only domains which might form the light-responsive complement of the LH2 antenna. The present work applies membrane fractionation, radiolabelling and LDS-PAGE techniques to investigate the response of R. sphaeroides to lowered light intensity. The kinetics underlying this adaptation to low light conditions were revealed by radiolabelling with the bacteriochlorophyll (bchl) biosynthetic precursor, delta-aminolevulinate, which allowed us to measure only the bchls synthesised after the light intensity shift. We show that (1) the increase in LH2 antenna size is mainly restricted to the mature ICM membrane fraction, and the antenna composition of the precursor upper pigmented band (UPB) membrane remains constant, (2) the precursor UPB membrane is enriched in bchl synthase, the terminal enzyme of the bchl biosynthetic pathway, and (3) the LH2 and the complexes of intermediate migration in LDS-PAGE exhibit completely different labelling kinetics. Thus, new photosynthetic complexes, mainly LH2, are synthesised and assembled at the membrane initiation UPB sites, where the LH2 rings pack between the rows of dimeric cores fostering new LH2-LH1 interactions. Mature membranes also assemble new LH2 rings, but in this case the 'sandwich' regions between the rows of core dimers are already fully occupied and the bulk antenna pool is the favoured location for these new LH2 complexes.

Isolation, size estimates, and spectral heterogeneity of an oligomeric series of light-harvesting 1 complexes from Rhodobacter sphaeroides

A series of light-harvesting 1 (LH1) complexes was isolated by lithium dodecyl sulfate-polyacrylamide gel electrophoresis at 4 degrees C from Rhodobacter sphaeroides M21, which lacks the peripheral light-harvesting 2 (LH2) complex. This ladder of LH1 bands was also demonstrated in the wild type, partially superimposed upon a smaller number of LH2 complexes. An assessment of electrophoretic mobility vs acrylamide concentration, in which the reaction center LM particle and annular LH1 and LH2 complexes were used as standards of known structure, indicated that the LH1 gel bands 2 to 10 represent regular oligomers of an alpha beta heterodimeric unit, that vary in size from (alpha beta)(2-3) to (alpha beta)(10-11). The isolated LH1 complexes exhibited oligomeric state dependent optical properties, characterized by red shifts in near-IR absorption and emission maxima at 77 K of approximately 6 nm as aggregate sizes increased from approximately 3 to 7-8 alpha beta-heterodimers, accompanied by shifts in highly polarized fluorescence from the blue to the red side of the absorption band. This has been explained by the oligomerization of heterodimers to form a curvilinear array of excitonically coupled chromophores, with the anisotropic long-wavelength component, designated originally as B896, corresponding to low energy excitonic transitions arising from interactions within inhomogeneous BChl clusters [Westerhuis et al. (1999) J. Phys. Chem. B 103, 7733-7742]. Differences in electrophoretic profiles of LH1 bands between strains M21 and M2192, an LH1-only strain that also lacks PufX, further suggested that the more rapidly migrating bands represent arced fragments of the curvilinear array of LH1 complexes thought to exist as a large closed circular structure only in the latter strain. The electrophoretic banding pattern also indicated that the LH1 complex may be located at the peripheries of dimeric intramembrane particle arrays seen in freeze-fracture replicas of tubular M21 membranes; the possible role for the PufX protein in the assembly of these structures is discussed.

Effects of phospholipase A2 digestion on the carotenoid and bacteriochlorophyll components of the light-harvesting complexes in Rhodobacter sphaeroides chromatophores

The instantaneous electrochromic response of carotenoids associated with the B800-850 light-harvesting complex of Rhodobacter sphaeroides has been used widely as an intrinsic probe of membrane potential. In the present study, the structural basis for this phenomenon was examined by phospholipase A2 digestion of chromatophores from R. sphaeroides strain NF57G, containing B800-850 as the sole pigment-protein complex. The major phospholipase-induced alterations of the overall carotenoid absorption spectrum were characterized by an absorbance loss and a blue shift that were accompanied by a decrease in absorbance at 800 nm and a red shift in the B850 absorbance band. In wild-type chromatophores, the electrochromic carotenoid response induced by both flash illumination and a K+ diffusion potential was diminished by approximately 60% after 1 h of digestion. The initial loss of the carotenoid response was correlated specifically to the hydrolysis of phosphatidylethanolamine, and was shown to arise from effects exerted directly upon the electrochromically active carotenoid pool, possibly by alterations in the spatial relationship between the field-sensitive carotenoids and the polarizing permanent field. In phospholipase A2-digested NF57G preparations in which the B800 band was diminished by nearly half and the carotenoid response was abolished, no significant changes in the efficiency of energy transfer from carotenoids to bacteriochlorophyll were detected at 77 K, suggesting that the electrochromically active carotenoids are not energetically linked to B800 bacteriochlorophyll.

Identification and solubilization of a signal peptidase from the phototrophic bacterium Rhodobacter capsulatus

In Gram-negative bacteria, exported proteins are synthesized with an amino-terminal signal sequence which is cleaved off by the signal peptidase during, or shortly after the translocation process. Here, we report the identification and solubilization of a signal peptidase from the phototrophic bacterium Rhodobacter capsulatus which cleaves homologous and heterologous precursor proteins at the authentic cleavage site. This signal peptidase is the first identified component of the R. capsulatus protein export machinery.

Development of a cell-free system to study the membrane assembly of photosynthetic proteins of Rhodobacter capsulatus

A cell-free translation system from the facultatively photoheterotrophic bacterium Rhodobacter capsulatus is described. Synthesis of two proteins of the bacterium's photosynthetic apparatus (light-harvesting complex B870 alpha and beta) was performed by SP6 polymerase transcription of the subcloned genes, isolation of the mRNA and translation in vitro using a cell-free extract of R. capsulatus cells. The integration of these proteins in vitro into added intracytoplasmic membrane vesicles (ICM) is demonstrated. Without addition of ICM approximately 70% of the synthesized B870 proteins were soluble. If, however, ICM were present during synthesis, the majority of the soluble protein was found to associate with the membranes. The membrane-associated polypeptides could be solubilized only by detergent treatment but could not be extracted by treatment at alkaline pH (Na2CO3), suggesting that the proteins had been firmly inserted into the lipid bilayer. Moreover, the B870 alpha and beta proteins that integrated in vitro into ICM were also found to associate with pigment ligands and to assemble into a native reaction center/B870 complex. The native conformation of this complex isolated from ICM by Triton fractionation was demonstrated by microspectral analysis of the bound pigments.

Characterization and localization of phosphatidylglycerophosphate and phosphatidylserine synthases in Rhodobacter sphaeroides

Catalytic properties and membrane associations of the phosphatidylglycerophosphate (PGP) and phosphatidylserine (PS) synthases of Rhodobacter sphaeroides were examined to further characterize sites of phospholipid biosynthesis. In preparations of cytoplasmic membrane (CM) enriched in these activities, apparent Km values of PGP synthase were 90 microM for sn-glycerol-3-phosphate and 60 microM for CDP-diacylglycerol; the apparent Km of PS synthase for L-serine was near 165 microM. Both enzymes required Triton X-100 with optimal PS synthase activity at a detergent/CDP-diacylglycerol (mol/mol) ratio of 7.5:1.0, while for optimal PGP synthase, a range of 10-50:1.0 was observed. Unlike the enzyme in Escherichia coli and several other Gram-negative bacteria, the PS synthase activity had a specific requirement for magnesium and was tightly associated with membranes rather than ribosomes in crude cell extracts. Sedimentation studies suggested that the PGP synthase was distributed uniformly over the CM in both chemoheterotrophically and photoheterotrophically grown cells, while the PS synthase was confined mainly to a vesicular CM fraction. Solubilized PGP synthase activity migrated as a single band with a pI value near 5.5 in a chromato-focusing column and 5.8 on isoelectric focusing; in the latter procedure, the pI was shifted to 5.3 in the presence of CDP-diacylglycerol. The PGP synthase activity gave rise to a single polypeptide band in lithium dodecyl sulfate-polyacrylamide gel electrophoresis at 4 degrees C.

Regulation of tetrapyrrole synthesis by light in chemostat cultures of Rhodobacter sphaeroides

Control of bacteriochlorophyll (Bchl), magnesium protoporphyrin monomethyl ester (MgPME), cytochromes, and coproporphyrin by light was studied with chemostat cultures of Rhodobacter sphaeroides growing at a constant dilution rate. By increasing the growth-limiting light energy flux from 10 to 55 W/m2, specific Bchl contents decreased from 19.3 to 7.9 nmol/mg of protein. This was strictly proportional to a decrease in the ratio of B800-850 to B875 light-harvesting complexes. MgPME levels increased from 1.5 to 5.3 nmol/mg of protein, while cytochrome as well as coproporphyrin levels stayed constant at 0.46 and 1.95 nmol/mg of protein, respectively. Since in chemostat cultures steady-state levels of a product represent the rate of synthesis, these results infer only slight control of the rate-limiting step of total tetrapyrrol formation by light. In substrate-limited cultures MgPME was accumulated when growth and Bchl formation approached substrate saturation. This suggests that light controls a second step, i.e., MgPME conversion, whenever too much precursor is available, owing to the low sensitivity of the initial step of control. MgPME was preferentially localized in a subcellular fraction with high contents of B875 complexes. A second fraction exhibiting increased contents of B800-850 complexes lacked significant levels of MgPME. These results are discussed in terms of localization of Bchl synthesis in the membrane system of R. sphaeroides.