Rhodopseudomonas sphaeroides membranes: alterations in phospholipid composition in aerobically and phototrophically grown cells

Production of long-chain free fatty acids from metabolically engineered Rhodobacter sphaeroides heterologously producing periplasmic phospholipase A2 in dodecane-overlaid two-phase culture

Background

Long-chain free fatty acids (FFAs) are a type of backbone molecule that can react with alcohol to produce biodiesels. Various microorganisms have become potent producers of FFAs. Efforts have focused on increasing metabolic flux to the synthesis of either neutral fat or fatty acyl intermediates attached to acyl carrier protein (ACP), which are the source of FFAs. Membrane lipids are also a source of FFAs. As an alternative way of producing FFAs, exogenous phospholipase may be used after heterologous production and localization in the periplasmic space. In this work, we examined whether Rhodobacter sphaeroides, which forms an intracytoplasmic membrane, can be used for long-chain FFA production using phospholipase.

Results

The recombinant R. sphaeroides strain Rs-A2, which heterologously produces Arabidopsis thaliana phospholipase A2 (PLA2) in the periplasm, excretes FFAs during growth. FFA productivity under photoheterotrophic conditions is higher than that observed under aerobic or semiaerobic conditions. When the biosynthetic enzymes for FA (β-ketoacyl-ACP synthase, FabH) and phosphatidate (1-acyl-sn-glycerol-3-phosphate acyltransferase, PlsC) were overproduced in Rs-A2, the FFA productivity of the resulting strain Rs-HCA2 was elevated, and the FFAs produced mainly consisted of long-chain FAs of cis-vaccenate, stearate, and palmitate in an approximately equimolar ratio. The high-cell-density culture of Rs-HCA2 with DMSO in two-phase culture with dodecane resulted in an increase of overall carbon substrate consumption, which subsequently leads to a large increase in FFA productivity of up to 2.0 g L⁻¹ day⁻¹. Overexpression of the genes encoding phosphate acyltransferase (PlsX) and glycerol-3-phosphate acyltransferase (PlsY), which catalyze the biosynthetic steps immediately upstream from PlsC, in Rs-HCA2 generated Rs-HXYCA2, which grew faster than Rs-HCA2 and showed an FFA productivity of 2.8 g L⁻¹ day⁻¹ with an FFA titer of 8.5 g L⁻¹.

Conclusion

We showed that long-chain FFAs can be produced from metabolically engineered R. sphaeroides heterologously producing PLA2 in the periplasm. The FFA productivity was greatly increased by high-cell-density culture in two-phase culture with dodecane. This approach provides highly competitive productivity of long-chain FFAs by R. sphaeroides compared with other bacteria. This method may be applied to FFA production by other photosynthetic bacteria with similar differentiated membrane systems.

Electronic supplementary material

The online version of this article (10.1186/s12934-019-1070-8) contains supplementary material, which is available to authorized users.

The effectiveness of styrene-maleic acid (SMA) copolymers for solubilisation of integral membrane proteins from SMA-accessible and SMA-resistant membranes

Solubilisation of biological lipid bilayer membranes for analysis of their protein complement has traditionally been carried out using detergents, but there is increasing interest in the use of amphiphilic copolymers such as styrene maleic acid (SMA) for the solubilisation, purification and characterisation of integral membrane proteins in the form of protein/lipid nanodiscs. Here we survey the effectiveness of various commercially-available formulations of the SMA copolymer in solubilising Rhodobacter sphaeroides reaction centres (RCs) from photosynthetic membranes. We find that formulations of SMA with a 2:1 or 3:1 ratio of styrene to maleic acid are almost as effective as detergent in solubilising RCs, with the best solubilisation by short chain variants (<30kDa weight average molecular weight). The effectiveness of 10kDa 2:1 and 3:1 formulations of SMA to solubilise RCs gradually declined when genetically-encoded coiled-coil bundles were used to artificially tether normally monomeric RCs into dimeric, trimeric and tetrameric multimers. The ability of SMA to solubilise reaction centre-light harvesting 1 (RC-LH1) complexes from densely packed and highly ordered photosynthetic membranes was uniformly low, but could be increased through a variety of treatments to increase the lipid:protein ratio. However, proteins isolated from such membranes comprised clusters of complexes in small membrane patches rather than individual proteins. We conclude that short-chain 2:1 and 3:1 formulations of SMA are the most effective in solubilising integral membrane proteins, but that solubilisation efficiencies are strongly influenced by the size of the target protein and the density of packing of proteins in the membrane.

Oxygen-dependent regulation of bacterial lipid production

Understanding the mechanisms of lipid accumulation in microorganisms is important for several reasons. In addition to providing insight into assembly of biological membranes, lipid accumulation has important applications in the production of renewable fuels and chemicals. The photosynthetic bacterium Rhodobacter sphaeroides is an attractive organism to study lipid accumulation, as it has the ability to increase membrane production at low O2 tensions. Under these conditions, R. sphaeroides develops invaginations of the cytoplasmic membrane to increase its membrane surface area for housing of the membrane-bound components of its photosynthetic apparatus. Here we use fatty acid levels as a reporter of membrane lipid content. We show that, under low-O2 and anaerobic conditions, the total fatty acid content per cell increases 3-fold. We also find that the increases in the amount of fatty acid and photosynthetic pigment per cell are correlated as O2 tensions or light intensity are changed. To ask if lipid and pigment accumulation were genetically separable, we analyzed strains with mutations in known photosynthetic regulatory pathways. While a strain lacking AppA failed to induce photosynthetic pigment-protein complex accumulation, it increased fatty acid content under low-O2 conditions. We also found that an intact PrrBA pathway is required for low-O2-induced fatty acid accumulation. Our findings suggest a previously unknown role of R. sphaeroides transcriptional regulators in increasing fatty acid and phospholipid accumulation in response to decreased O2 tension.

IMPORTANCE

Lipids serve important functions in living systems, either as structural components of membranes or as a form of carbon storage. Understanding the mechanisms of lipid accumulation in microorganisms is important for providing insight into the assembly of biological membranes and additionally has important applications in the production of renewable fuels and chemicals. In this study, we investigate the ability of Rhodobacter sphaeroides to increase membrane production at low O2 tensions in order to house its photosynthetic apparatus. We demonstrate that this bacterium has a mechanism to increase lipid content in response to decreased O2 tension and identify a transcription factor necessary for this response. This is significant because it identifies a transcriptional regulatory pathway that can increase microbial lipid content.

Bacterial reaction centers purified with styrene maleic acid copolymer retain native membrane functional properties and display enhanced stability

Integral membrane proteins often present daunting challenges for biophysical characterization, a fundamental issue being how to select a surfactant that will optimally preserve the individual structure and functional properties of a given membrane protein. Bacterial reaction centers offer a rare opportunity to compare the properties of an integral membrane protein in different artificial lipid/surfactant environments with those in the native bilayer. Here, we demonstrate that reaction centers purified using a styrene maleic acid copolymer remain associated with a complement of native lipids and do not display the modified functional properties that typically result from detergent solubilization. Direct comparisons show that reaction centers are more stable in this copolymer/lipid environment than in a detergent micelle or even in the native membrane, suggesting a promising new route to exploitation of such photovoltaic integral membrane proteins in device applications.

Study of effect of molecular mobility in chromatophore membranes of the bacterium E. shaposhnikovii on processes of photoinduced electron transport using the NMR-spin-echo method with isotope substitution and dehydration

The effect of dehydration and (2)H2O/H2O isotope substitution on electron transport reactions and relaxation of proton-containing groups was studied in chromatophore membranes of Ectothiorhodospira shaposhnikovii. During dehydration (including isotope substitution of hydrate water) of preliminarily dehydrated isolated photosynthetic membranes there was a partial correlation between hydration intervals within which activation of electron transport from high-potential cytochrome c to photoactive bacteriochlorophyll dimer P890 of photosynthetic reaction center and variation of spin-lattice and spin-spin proton relaxation time was observed. Partial correlation between hydration intervals can be considered as evidence of correlation between mobility of non-water proton-containing groups with proton relaxation frequency approximately 10(8) sec(-1) with efficiency of electron transfer at the donor side of the chain.

Membrane curvature induced by aggregates of LH2s and monomeric LH1s

The photosynthetic apparatus of purple bacteria is contained within organelles called chromatophores, which form as extensions of the cytoplasmic membrane. The shape of these chromatophores can be spherical (as in Rhodobacter sphaeroides), lamellar (as in Rhodopseudomonas acidophila and Phaeospirillum molischianum), or tubular (as in certain Rb. sphaeroides mutants). Chromatophore shape is thought to be influenced by the integral membrane proteins Light Harvesting Complexes I and II (LH1 and LH2), which pack tightly together in the chromatophore. It has been suggested that the shape of LH2, together with its close packing in the membrane, induces membrane curvature. The mechanism of LH2-induced curvature is explored via molecular dynamics simulations of multiple LH2 complexes in a membrane patch. LH2s from three species-Rb. sphaeroides, Rps. acidophila, and Phsp. molischianum-were simulated in different packing arrangements. In each case, the LH2s pack together and tilt with respect to neighboring LH2s in a way that produces an overall curvature. This curvature appears to be driven by a combination of LH2's shape and electrostatic forces that are modulated by the presence of well-conserved cytoplasmic charged residues, the removal of which inhibits LH2 curvature. The interaction of LH2s and an LH1 monomer is also explored, and it suggests that curvature is diminished by the presence of LH1 monomers. The implications of our results for chromatophore shape are discussed.

Mutation of a single residue, beta-glutamate-20, alters protein-lipid interactions of light harvesting complex II

It is well established that assembly of the peripheral antenna complex, LH2, is required for proper photosynthetic membrane biogenesis in the purple bacterium Rhodobacter sphaeroides. The underlying interactions are, as yet, not understood. Here we examined the relationship between the morphology of the photosynthetic membrane and the lipid–protein interactions at the LH2–lipid interface. The non-bilayer lipid, phosphatidylethanolamine, is shown to be highly enriched in the boundary lipid phase of LH2. Sequence alignments indicate a putative lipid binding site, which includes β-glutamate-20 and the adjacent carotenoid end group. Replacement of β-glutamate-20 with alanine results in significant reduction of phosphatidylethanolamine and concomitant raise in phosphatidylcholine in the boundary lipid phase of LH2 without altering the lipid composition of the bulk phase. The morphology of the LH2 housing membrane is, however, unaffected by the amino acid replacement. In contrast, simultaneous modification of glutamate-20 and exchange of the carotenoid sphaeroidenone with neurosporene results in significant enlargement of the vesicular membrane invaginations. These findings suggest that the LH2 complex, specifically β-glutamate-20 and the carotenoids' polar head group, contribute to the shaping of the photosynthetic membrane by specific interactions with surrounding lipid molecules.

Stabilization of charge separation and cardiolipin confinement in antenna-reaction center complexes purified from Rhodobacter sphaeroides

The reaction center-light harvesting complex 1 (RC-LH1) purified from the photosynthetic bacterium Rhodobacter sphaeroides has been studied with respect to the kinetics of charge recombination and to the phospholipid and ubiquinone (UQ) complements tightly associated with it. In the antenna-RC complexes, at 6.5<pH<9.0, P(+)Q(B)(-) recombines with a pH independent average rate constant <k> more than three times smaller than that measured in LH1-deprived RCs. At increasing pH values, for which <k> increases, the deceleration observed in RC-LH1 complexes is reduced, vanishing at pH >11.0. In both systems kinetics are described by a continuous rate distribution, which broadens at pH >9.5, revealing a strong kinetic heterogeneity, more pronounced in the RC-LH1 complex. In the presence of the antenna the Q(A)Q(B)(-) state is stabilized by about 40 meV at 6.5<pH<9.0, while it is destabilized at pH >11. The phospholipid/RC and UQ/RC ratios have been compared in chromatophore membranes, in RC-LH1 complexes and in the isolated peripheral antenna (LH2). The UQ concentration in the lipid phase of the RC-LH1 complexes is about one order of magnitude larger than the average concentration in chromatophores and in LH2 complexes. Following detergent washing RC-LH1 complexes retain 80-90 phospholipid and 10-15 ubiquinone molecules per monomer. The fractional composition of the lipid domain tightly bound to the RC-LH1 (determined by TLC and (31)P-NMR) differs markedly from that of chromatophores and of the peripheral antenna. The content of cardiolipin, close to 10% weight in chromatophores and LH2 complexes, becomes dominant in the RC-LH1 complexes. We propose that the quinone and cardiolipin confinement observed in core complexes reflects the in vivo heterogeneous distributions of these components. Stabilization of the charge separated state in the RC-LH1 complexes is tentatively ascribed to local electrostatic perturbations due to cardiolipin.

Enthalpy/entropy driven activation of the first interquinone electron transfer in bacterial photosynthetic reaction centers embedded in vesicles of physiologically important phospholipids

The thermodynamics and kinetics of light-induced electron transfer in bacterial photosynthetic RCs are sensitive to physiologically important lipids (phosphatidylcholine, cardiolipin and phosphatidylglycerol) in the environment. The analysis of the temperature-dependence of the rate of the P(+)Q(A)(-)Q(B)-->P(+)Q(A)Q(B)(-) interquinone electron transfer revealed high enthalpy change of activation in zwitterionic or neutral micelles and vesicles and low enthalpy change of activation in vesicles constituted of negatively charged phospholipids. The entropy change of activation was compensated by the changes of enthalpy, thus the free energy change of activation ( approximately 500 meV) did not show large variation in vesicles of different lipids.

Protein/Lipid Interaction in the Bacterial Photosynthetic Reaction Center: Phosphatidylcholine and Phosphatidylglycerol Modify the Free Energy Levels of the Quinones

The role of characteristic phospholipids of native membranes, phosphatidylcholine (PC), phosphatidylglycerol (PG), and cardiolipin (CL), was studied in the energetics of the acceptor quinone side in photosynthetic reaction centers of Rhodobacter sphaeroides. The rates of the first, k(AB)(1), and the second, k(AB)(2), electron transfer and that of the charge recombination, k(BP), the free energy levels of Q(A)(-)Q(B) and Q(A)Q(B)(-) states, and the changes of charge compensating protein relaxation were determined in RCs incorporated into artificial lipid bilayer membranes. In RCs embedded in the PC vesicle, k(AB)(1) and k(AB)(2) increased (from 3100 to 4100 s(-1) and from 740 to 3300 s(-1), respectively) and k(BP) decreased (from 0.77 to 0.39 s(-1)) compared to those measured in detergent at pH 7. In PG, k(AB)(1) and k(BP) decreased (to values of 710 and 0.26 s(-1), respectively), while k(AB)(2) increased to 1506 s(-1) at pH 7. The free energy between the Q(A)(-)Q(B) and Q(A)Q(B)(-) states decreased in PC and PG (DeltaG degrees (Q)A-(Q)B(-->)(Q)A(Q)B- = -76.9 and -88.5 meV, respectively) compared to that measured in detergent (-61.8 meV). The changes of the Q(A)/Q(A)(-) redox potential measured by delayed luminescence showed (1) a differential effect of lipids whether RC incorporated in micelles or vesicles, (2) an altered binding interaction between anionic lipids and RC, (3) a direct influence of PC and PG on the free energy levels of the primary and secondary quinones probably through the intraprotein hydrogen-bonding network, and (4) a larger increase of the Q(A)/Q(A)(-) free energy in PG than in PC both in detergent micelles and in single-component vesicles. On the basis of recent structural data, implications of the binding properties of phospholipids to RC and possible interactions between lipids and electron transfer components will be 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.

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.

Lipid biosynthesis in synchronized cultures of the photosynthetic bacterium Rhodopseudomonas sphaeroides

Lipid biosynthesis has been studied in photosynthetic cultures of Rhodopseudomonas sphaeroides that had been synchronized by stationary-phase cycling or by a centrifugation selection procedure. Synchrony index values in the range 0.70-0.80 were obtained for the first cell cycle with both synchronization methods. The major membrane lipids phosphatidylethanolamine and phosphatidylglycerol were accumulated discontinuously during the cell cycle, their mass doubling immediately before cell division. This accumulation of lipid corresponded to peaks in incorporation of radioactivity from either [1-14C]acetate or [2-3H]glycerol into individual acyl lipids as measured in individual portions of bacteria. For phosphatidylglycerol an additional peak of incorporation of radioactivity from [2-3H]glycerol was found midway through the cell cycle. In spite of their rather similar endogenous fatty acid compositions, the individual phosphoacylglycerols showed distinctive patterns of incorporation of radioactivity from [1-14C]acetate into their acyl moieties. The discontinuous synthesis of acyl lipids observed in cultures of Rhodopseudomonas sphaeroides synchronized by either stationary-phase cycling or centrifugation selection procedures contrasted with the accumulation of chlorophyll-protein complexes whose amounts were found to increase throughout the cell cycle. The implications of these findings for the control of lipid synthesis in bacterial photosynthetic membranes are discussed.

Sites of phospholipid biosynthesis during induction of intracytoplasmic membrane formation in Rhodopseudomonas sphaeroides

A rapid, gratuitous and cell-division uncoupled induction of intracytoplasmic photosynthetic membrane formation was demonstrated in low-aeration suspensions of chemotrophically grown Rhodopseudomonas sphaeroides. Despite a nearly 2-fold increase in phospholipid levels, no significant increases were detected in the specific activities of CDP-1,2-diacyl-sn-glycerol:sn-glycerol-3-phosphate phosphatidyltransferase (phosphatidylglycerophosphate synthase, EC 2.7.8.5) and CDP-1,2-diacyl-sn-glycerol:L-serine O-phosphatidyltransferase (phosphatidylserine synthase, EC 2.7.8.8), the first committed enzymes of anionic and zwitterionic phospholipid biosyntheses, respectively. The distribution of phosphatidylglycerophosphate and phosphatidylserine synthase activities after rate-zone sedimentation of cell-free extracts indicated that intracytoplasmic membrane phospholipids were synthesized mainly within distinct domains of the conserved cytoplasmic membrane. Labeling studies with 32Pi and L-[3H]phenylalanine suggested that preexisting phospholipid was utilized initially as the matrix for insertion of intracytoplasmic membrane protein that was synthesized and assembled de novo during induction.

Intracellular localization of photosynthetic membrane growth initiation sites in Rhodopseudomonas sphaeroides

Putative membrane invagination sites at which intracytoplasmic photosynthetic membrane growth is initiated in Rhodopseudomonas sphaeroides can be isolated in an upper pigmented fraction by rate-zone sedimentation. The intracellular localization of membranes present in the isolated fraction was investigated with the impermeant surface-labeling reagent pyridoxal 5'-phosphate, which has been shown to diffuse into the periplasmic space and to label proteins of both the peripheral cytoplasmic membrane and the mature intracytoplasmic membrane. A comparison of the extent of labeling at 25 and 0 degrees C was consistent with the possibility that membranes present in the upper pigmented fraction arise from sites near the cell periphery. Pronase digestion of the surface-labeled membranes suggested further that the purified upper fraction consisted largely of open membrane fragments and that the majority of the intracytoplasmic membrane is labeled by this procedure. The pigmented membrane growth initiation sites were separated partially from undifferentiated respiratory cytoplasmic membrane also present in the upper fraction.

In vivo metabolic intermediates of phospholipid biosynthesis in Rhodopseudomonas sphaeroides

The in vivo metabolic pathways of phospholipid biosynthesis in Rhodopseudomonas sphaeroides have been investigated. Rapid pulse-chase-labeling studies indicated that phosphatidylethanolamine and phosphatidylglycerol were synthesized as in other eubacteria. The labeling pattern observed for N-acylphosphatidylserine (NAPS) was inconsistent with the synthesis of this phospholipid occurring by direct acylation of phosphatidylserine (PS). Rather, NAPS appeared to be kinetically derived from an earlier intermediate such as phosphatidic acid or more likely CDP-diglyceride. Tris-induced NAPS accumulation specifically reduced the synthesis of PS. Treatment of cells with a bacteriostatic concentration of hydroxylamine (10 mM) greatly reduced total cellular phospholipid synthesis, resulted in accumulation of PS, and stimulated the phosphatidylglycerol branch of phospholipid metabolism relative to the PS branch of the pathway. When the cells were treated with a lower hydroxylamine dosage (50 microM), total phospholipid synthesis lagged as PS accumulated, however, phospholipid synthesis resumed coincident with a reversal of PS accumulation. Hydroxylamine alone was not sufficient to promote NAPS accumulation but this compound allowed continued NAPS accumulation when cells were grown in medium containing Tris. The significance of these observations is discussed in terms of NAPS biosynthesis being representative of a previously undescribed branch of the phospholipid biosynthetic sequence.

Kinetic analysis of N-acylphosphatidylserine accumulation and implications for membrane assembly in Rhodopseudomonas sphaeroides

The accumulation of N-acylphosphatidylserine (NAPS) in response to the inclusion of Tris in the growth medium of Rhodopseudomonas sphaeroides strain M29-5 has been examined. In the accompanying paper (Donohue et al., J. Bacteriol. 152:000--000, 1982), we show that in response to Tris, NAPS accumulated to as much as 40% of the total cellular phospholipid content. NAPS accumulation began immediately upon addition of Tris and was reflected as an abrupt 12-fold increase in the apparent rate of NAPS accumulation. We suggest that Tris altered the flow of metabolites through a preexisting and previously unknown metabolic pathway. NAPS accumulation ceased immediately upon the removal of Tris; however, accumulated NAPS remained largely metabolically stable. Importantly, under conditions in which NAPS was not accumulated, the intracytoplasmic membrane was shown to be virtually devoid of newly synthesized NAPS. The significance of this observation is discussed in terms of its physiological implications on phospholipid transfer and membrane biogenesis in R. sphaeroides.

Alterations in the phospholipid composition of Rhodopseudomonas sphaeroides and other bacteria induced by Tris

Alterations in the phospholipid head group composition of most strains of Rhodopseudomonas sphaeroides, as well as Rhodopseudomonas capsulata and Paracoccus denitrificans, occurred when cells were grown in medium supplemented with Tris. Growth of R. sphaeroides M29-5 in Tris-supplemented medium resulted in the accumulation of N-acylphosphatidylserine (NAPS) to as much as 40% of the total whole-cell phospholipid, whereas NAPS represented approximately 28 an 33% of the total phospholipid when R. capsulata and P. denitrificans respectively, were grown in medium containing 20 mM Tris. The accumulation of NAPS occurred primarily at the expense of phosphatidylethanolamine in both whole cells and isolated membranes of R. sphaeroides and had no detectable effect on cell growth under either chemoheterotrophic or photoheterotrophic conditions. Yeast extract (0.1%) and Casamino Acids (1.0%) were found to be antagonistic to the Tris-induced (20 mM) alteration in the phospholipid composition of R. sphaeroides. The wild-type strains R. sphaeroides 2.4.1 and RS2 showed no alteration in their phospholipid composition when they were grown in medium supplemented with Tris. In all strains of Rhodospirillaceae tested, as well as in P. denitrificans, NAPS represented between 1.0 and 2.0% of the total phospholipid when cells were grown in the absence of Tris. [32P]orthophosphoric acid entered NAPS rapidly in strains of R. sphaeroides that do (strain M29-5) and do not (strain 2.4.1) accumulate this phospholipid in response to Tris. Our data indicate that the phospholipid head group composition of many Rhodospirillaceae strains, as well as P. denitrificans, is easily manipulated; thus, these bacteria may provide good model systems for studying the effects of these modifications on membrane structure and function in a relatively unperturbed physiological system.