How is leghemoglobin involved in peribacteroid membrane degradation during nodule senescence?

An increase in the rate of succinate and glutamate uptake by isolated symbiosomes from French bean nodules was observed in the presence of iron plus H2O2. The lipid bilayer, and not proteins involved in transport, seems to be the major target of radical attack. Leghemoglobin in the presence of a 6-fold excess of H2O2 (where heme breakdown and iron release occurred) provoked also an increase in peribacteroid membrane permeability. In contrast, this hemoprotein in the presence of a 2-fold excess of H2O2 (where a protein radical was generated) was without effect. We suggest that in vivo the release of heme iron may constitute the major process concerning the involvement of leghemoglobin in the degradation of the peribacteroid membrane during nodule senescence.

Cloning of ndhK from soybean chloroplasts using antibodies raised to mitochondrial complex I

A soybean shoot cDNA expression library was screened with polyclonal antibodies raised against red beet complex I and several clones were identified. One clone, consisting of a 1 kb insert, was fully sequenced. The sequence of 1025 bp was found to contain two extended open reading frames and the proteins encoded were identified as the ndhK and ndhJ products of the chloroplast genome. Nuclear, mitochondrial and chloroplast DNA was isolated and probed with a ndhK-specific probe. The chloroplast DNA contained a single copy of the cloned insert. With nuclear DNA, positively hybridising bands of 1.2, 2.7 and 3.2 kb were observed indicating that at least one gene homologous to ndhK of the chloroplast genome, is also present in the nucleus. The mitochondrial DNA did not hybridise with the ndhK probe. Western analysis of thylakoid proteins with the mitochondrial complex I antibodies revealed several bands. It is suggested that soybean contains two copies of the ndhK gene, one, on the plastid genome, coding for a subunit of a chloroplast NAD(P)H dehydrogenase, and the other, in the nucleus, coding for a subunit of mitochondrial complex I.

Isolation and characterization of a ntrC mutant of Bradyrhizobium (Parasponia) sp. ANU289

A mutant of Bradyrhizobium (Parasponia) sp. ANU289 affected in the regulation of nitrogen metabolism was isolated. The mutant, designated ANU293, was unable to induce ammonium transport (Amt), nitrate reductase (NR) or glutamine synthetase II (GSII) activities under conditions that induce these activities in the wild-type. However, glutamine synthetase I (GSI), which is expressed constitutively in the wild-type, was present at normal levels in the mutant. The mutant also retained the ability to fix nitrogen in vitro and in planta, although nodule development on siratro (Macroptilium atropurpureum) was retarded. Southern blot analysis showed that ntrC, the product of which is involved in regulation of nitrogen metabolism, is the site of pSUP1021 insertion in ANU293. These results indicate that the transcriptional activator NtrC is required for the expression of Amt, NR and GSII, but not GSI or nitrogenase in Bradyrhizobium (Parasponia) sp. ANU289.

Matrix NADH dehydrogenases of plant mitochondria and sites of quinone reduction by complex I

In order to distinguish the pathways involved in the oxidation of matrix NADH in plant mitochondria, the oxidation of NADH and nicotinamide hypoxanthine dinucleotide (reduced form) was investigated in submitochondrial particles prepared from beetroot (Beta vulgaris L. cv. Derwent Globe) and soybeans (Glycine max L. cv. Bragg). Nicotinamide-hypoxanthine-dinucleotide(reduced form)-oxidase activity was more strongly inhibited by rotenone than the NADH-oxidase activity but both of the rotenone-inhibited activities could be stimulated by adding ubiquinone-1. The corresponding ubiquinone-1-reductase activities were inhibited by rotenone (to 69%) and further inhibited by N,N'-dicyclohexylcarbodiimide (to 79%), whilst the K3Fe(CN)6-reductase activities were not sensitive to either rotenone or N,N'-dicyclohexylcarbodiimide. Immunological analysis of mitochondrial proteins using an antiserum raised against purified beetroot complex I indicated very few differences between soybean and fresh and aged beetroot mitochondria, despite their varying sensitivities to rotenone. We confirm that there are two dehydrogenases capable of oxidising internal NADH and that only one of these, namely complex I, is inhibited by rotenone. Further, we conclude that complex I has two potential sites of quinone reduction, both sensitive to N,N'-dicyclohexycarbodiimide inhibition but only one of which is sensitive to rotenone inhibition.

Tissue-specific expression of the alternative oxidase in soybean and siratro

Alternative oxidase activity (cyanide-insensitive respiration) was measured in mitochondria from the shoots, roots, and nodules of soybean (Glycine max L.) and siratro (Macroptilium atropurpureum) plants. Activity was highest in the shoots and lowest in the nodules. Alternative oxidase activity was associated with one (roots) or two (shoots) proteins between 30 and 35 kilodaltons that were detected by western blotting with a monoclonal antibody against Sauromatum guttatum alternative oxidase. No such protein was detected in nodule mitochondria. Measurements of oxygen uptake by isolated soybean root and nodule cells in the presence of cyanide and salicylhydroxamic acid indicated that alternative oxidase activity was confined to the uninfected cortex cells of the nodule. Immunoprecipitation of translation products of mRNA isolated from soybean shoots revealed a major band at 43 kilodaltons that is assumed to be the precursor of an alternative oxidase protein. This band was not seen when mRNA from nodules was treated in the same fashion. The results indicate that tissue-specific expression of the alternative oxidase occurs in soybean and siratro.

Protein phosphorylation stimulates the rate of malate uptake across the peribacteroid membrane of soybean nodules

Incubation of intact isolated symbiosomes with [gamma-32P]ATP, followed by isolation of the peribacteroid membrane and polypeptide analysis, showed that a single major polypeptide at 26 kDa was labelled. Antibodies raised against nodulin 26 reacted with a similar sized polypeptide. Incubation of the symbiosomes with alkaline phosphatase removed the label from this polypeptide. Pre-incubation with ATP stimulated malate accumulation by isolated symbiosomes, but only slightly (10-30%). Pre-treatment of symbiosomes with alkaline phosphatase inhibited malate uptake substantially and this inhibition was completely relieved by addition of ATP. The ATP stimulation of malate uptake was not affected by ATPase inhibitors. It is suggested that the rate of malate uptake across the peribacteroid membrane is controlled by phosphorylation of nodulin 26.

Regulation of Alternative Pathway Activity in Plant Mitochondria : Deviations from Q-Pool Behavior during Oxidation of NADH and Quinols

External NADH and succinate were oxidized at similar rates by soybean (Glycine max) cotyledon and leaf mitochondria when the cytochrome chain was operating, but the rate of NADH oxidation via the alternative oxidase was only half that of succinate. However, measurements of the redox poise of the endogenous quinone pool and reduction of added quinones revealed that external NADH reduced them to the same, or greater, extent than did succinate. A kinetic analysis of the relationship between alternative oxidase activity and the redox state of ubiquinone indicated that the degree of ubiquinone reduction during external NADH oxidation was sufficient to fully engage the alternative oxidase. Measurements of NADH oxidation in the presence of succinate showed that the two substrates competed for cytochrome chain activity but not for alternative oxidase activity. Both reduced Q-1 and duroquinone were readily oxidized by the cytochrome oxidase pathway but only slowly by the alternative oxidase pathway in soybean mitochondria. In mitochondria isolated from the thermogenic spadix of Philodendron selloum, on the other hand, quinol oxidation via the alternative oxidase was relatively rapid; in these mitochondria, external NADH was also oxidized readily by the alternative oxidase. Antibodies raised against alternative oxidase proteins from Sauromatum guttatum cross-reacted with proteins of similar molecular size from soybean mitochondria, indicating similarities between the two alternative oxidases. However, it appears that the organization of the respiratory chain in soybean is different, and we suggest that some segregation of electron transport chain components may exist in mitochondria from nonthermogenic plant tissues.

Specificity and regulation of the dicarboxylate carrier on the peribacteroid membrane of soybean nodules

Malate and succinate were taken up rapidly by isolated, intact peribacteroid units (PBUs) from soybean (Glycine max (L.) Merr.) root nodules and inhibited each other in a competitive manner. Malonate uptake was slower and was severely inhibited by equimolar malate in the reaction medium. The apparent Km for malonate uptake was higher than that for malate and succinate uptake. Malate uptake by PBUs was inhibited by (in diminishing order of severity) oxaloacetate, fumarate, succinate, phthalonate and oxoglutarate. Malonate and butylmalonate inhibited only slightly and pyruvate,isocitrate and glutamate not at all. Of these compounds, only oxaloacetate, fumarate and succinate inhibited malate uptake by free bacteroids. Malate uptake by PBUs was inhibited severely by the uncoupler carbonylcyanidem-chlorophenyl hydrazone and the respiratory poison KCN, and was stimulated by ATP. We conclude that the peribacteroid membrane contains a dicarboxylate transport system which is distinct from that on the bacteroid membrane and other plant membranes. This system can catalyse the rapid uptake of a range of dicarboxylates into PBUs, with malate and succinate preferred substrates, and is likely to play an important role in symbiotic nitrogen fixation. Energization of both the bacteroid and peribacteroid membranes controls the rate of dicarboxylate transport into peribacteroid units.

Ammonia (C-Methylamine) Transport across the Bacteroid and Peribacteroid Membranes of Soybean Root Nodules

[(14)C]Methylamine (MA; an analog of ammonia) was used to investigate ammonia transport across the bacteroid and peribacteroid membranes (PBM) from soybean (Glycine max) root nodules. Free-living Bradyrhizobium japonicum USDA110 grown under nitrogen-limited conditions showed rapid MA uptake with saturation kinetics at neutral pH, indicative of a carrier. Exchange of accumulated MA for added ammonia occurred, showing that the carrier recognized both NH(4) (+) and CH(3)NH(3) (+). MA uptake by isolated bacteroids, on the other hand, was very slow at low concentrations of MA and increased linearly with increasing MA concentration up to 1 millimolar. Ammonia did not inhibit MA by isolated bacteroids and did not cause efflux of accumulated MA. PBM-enclosed bacteroids (peribacteroid units [PBUs]) were qualitatively similar to free bacteroids with respect to MA transport. The rates of uptake and efflux of MA by PBUs were linearly dependent on the imposed concentration gradient and unaffected by NH(4)Cl. MA uptake by PBUs increased exponentially with increasing pH, confirming that the rate increased linearly with increasing CH(3)NH(2) concentration. The results are consistent with other evidence that transfer of ammonia from the nitrogen-fixing bacteroid to the host cytosol in soybean root nodules occurs solely by simple diffusion of NH(3) across both the bacteroid and peribacteroid membranes.

Evidence for Metabolic Domains within the Matrix Compartment of Pea Leaf Mitochondria : Implications for Photorespiratory Metabolism

The simultaneous oxidation of malate and of glycine was investigated in pea (Pisum sativum) leaf mitochondria. Adding malate to state 4 glycine oxidation did not inhibit, and under some conditions stimulated, glycine oxidation. State 4 oxygen uptake with glycine is restricted because of the control exerted by the membrane potential but reoxidation of NADH by oxaloacetate reduction can still occur. Thus, malate addition stimulates glycine metabolism by producing oxaloacetate. The malate dehydrogenase (EC 1.1.1.37) enzyme fraction remote from glycine decarboxylase (EC 2.1.2.10) oxidizes malate whereas that closely associated with it produces malate, i.e. they function in opposite directions. It is shown that these opposing directions of malate dehydrogenase activity occur within the same mitochondrial matrix compartment and not in different mitochondrial populations. It is concluded that metabolic domains containing different complements of mitochondrial enzymes exist within the one mitochondrial matrix without physical barriers separating them. The differential spatial organization within the matrix may account for the previously reported limited access of some enzymes to the respiratory electron transport chain. The implications for leaf mitochondrial metabolism are discussed.

Regulation of alternative pathway activity in plant mitochondria: nonlinear relationship between electron flux and the redox poise of the quinone pool

The dependence of respiratory flux via the alternative pathway on the redox poise of the ubiquinone (Q) pool was investigated in soybean cotyledon mitochondria. A marked nonlinear relationship was observed between Q-pool reduction level and O2 uptake via the alternative oxidase. Significant engagement of the alternative pathway was not apparent until Q-pool reduction level reached 35-40% but increased disproportionately on further reduction. Similar results were obtained with electron donation from either Complex 1 or Complex 2. Close agreement was obtained over a range of experimental conditions between the estimated contribution of the alternative pathway to total respiratory flux, as measured with salicylhydroxamic acid, and that predicted from the redox poise of the Q-pool. These results are discussed in terms of existing models of the regulation of respiratory flux via the alternative pathway.

Electrogenic ATPase Activity on the Peribacteroid Membrane of Soybean (Glycine max L.) Root Nodules

Electrogenic ATPase activity on the peribacteroid membrane from soybean (Glycine max L. cv Bragg) root nodules is demonstrated. Membrane energization was monitored using suspensions of intact peribacteroid membrane-enclosed bacteroids (peribacteroid units; PBUs) and the fluorescent probe for membrane potential (DeltaPsi), bis-(3-phenyl-5-oxoisoxazol-4yl) pentamethine oxonol. Generation of a positive DeltaPsi across the peribacteroid membrane was dependent upon ATP, inhibited by N,N'-dicyclohexyl-carbodiimide and vanadate, but insensitive to N-ethylmaleimide, azide, cyanide, oligomycin, and ouabain. The results suggest the presence of a single, plasma membrane-like, electrogenic ATPase on the peribacteroid membrane. The protonophore, carbonyl-cyanide m-chlorophenyl hydrazone, completely dissipated the established membrane potential. The extent of reduction in the steady state membrane potential upon addition of ions was used to estimate the relative permeability of the peribacteroid membrane to anions. By this criterion, the relative rates of anion transport across the peribacteroid membrane were: NO(3) (-) > NO(2) (-) > Cl(-) > acetate(-) > malate(-). The observation that 10 millimolar NO(3) (-) completely dissipated the membrane potential was of particular interest in view of the fact that NO(3) (-) inhibits symbiotic nitrogen fixation. The possible function of the ATPase in symbiotic nitrogen fixation is discussed.

Regulation of nonphosphorylating electron transport pathways in soybean cotyledon mitochondria and its implications for fat metabolism

The respiration of mitochondria isolated from germinating soybean cotyledons was strongly resistant to antimycin and KCN. This oxygen uptake was not related to lipoxygenase which was not detectable in purified mitochondria. The antimycin-resistant rate of O(2) uptake was greatest with succinate as substrate and least with exogenous NADH. Succinate was the only single substrate whose oxidation was inhibited by salicyl hydroxamic acid alone, indicating engagement of the alternative oxidase. Concurrent oxidation of two or three substrates led to greater involvement of the alternative oxidase. Despite substantial rotenone-resistant O(2) uptake with NAD-linked substrates, respiratory control was observed in the presence of antimycin, indicating restriction of electron flow through complex I. Addition of succinate to mitochondria oxidizing NAD-linked substrates in state four stimulated O(2) uptake substantially, largely by engaging the alternative oxidase. We suggest that these properties of soybean cotyledon mitochondria would enable succinate received from the glyoxysome during lipid metabolism to be rapidly oxidized, even under a high cytosolic energy charge.

Regulation of the soybean-Rhizobium nodule symbiosis by shoot and root factors

The availability of soybean mutants with altered symbiotic properties allowed an investigation of the shoot or root control of the relevant phenotype. By means of grafts between these mutants and wild-type plants (cultivar Bragg and Williams), we demonstrated that supernodulation as well as hypernodulation (nitrate tolerance in nodulation and lack of autoregulation) is shoot controlled in two mutants (nts382 and nts1116) belonging most likely to two separate complementation groups. The supernodulation phenotype was expressed on roots of the parent cultivar Bragg as well as the roots of cultivar Williams. Likewise it was shown that non-nodulation (resistance to Bradyrhizobium) is root controlled in mutant nod49. The shoot control of nodule initiation is epistatically suppressed by the non-nodulation, root-expressed mutation. These findings suggest that different plant organs can influence the expression of the nodulation phenotype.

Hydroxamate-Stimulated O(2) Uptake in Roots of Pisum sativum and Zea mays, Mediated by a Peroxidase : Its Consequences for Respiration Measurements

Low concentrations of salicylhydroxamic acid (<5 millimolar) stimulate O(2) uptake in intact roots of Pisum sativum. We demonstrate that the hydroxamate-stimulated O(2) uptake does not reside in the mitochondria. We also show that the hydroxamate-stimulated O(2) uptake is due to the activation of a peroxidase catalyzing reduction of O(2). This peroxidase, which can use both NADH and NADPH as a substrate, is stimulated by low concentrations of monophenols, e.g. salicylhydroxamic acid and 2-methoxyphenol. It is inhibited by high (20 millimolar) concentrations of salicylhydroxamic acid, cyanide, and scavengers of the superoxide free radical ion, e.g. ascorbate, gentisic acid, and catechol. In the presence of gentisic acid, O(2) uptake by intact pea roots was no longer stimulated by low concentrations of salicylhydroxamic acid. The consequence of the present finding for in vivo respiration measurements is that the use of low concentrations of salicylhydroxamic acid and uncoupler is reliable only in the presence of a suitable superoxide free radical scavenger which prevents activation of the peroxidase. It also confirms that high concentrations of salicylhydroxamic acid (20-25 millimolar) can be safely used in short-term experiments to assess the activity of the alternative path in intact roots.

Stimulation of respiration and nitrogenase in bacteroids of Siratro (Macroptilium atropurpureum) by plant nodule cytosol

Nitrogenase activity (acetylene reduction) of isolated Siratro (Macroptilium atropurpureum) bacteroids was stimulated by addition of plant cytosol fractions which also preserved activity at high (up to 3%) O2 tensions. These effects were not due to leghaemoglobin. Boiling removed some, but not all, of the protective capacity of the cytosol. Heat treated cytosol substantially stimulated the respiration of siratro bacteroids. Of a wide variety of compounds tested, only ascorbate could mimic the cytosol. Ascorbate was present in the cytosol fraction, in significant quantities. The effect of ascorbate was evident at low O2 concentrations and in purified bacteroids, and was inhibited by cyanide. Siratro bacteroids appear to possess an oxidase which could serve a protective role in vivo.

Enzymes of ammonia assimilation and ureide biosynthesis in soybean nodules: effect of nitrate

The effect of nitrate on N(2) fixation and the assimilation of fixed N(2) in legume nodules was investigated by supplying nitrate to well established soybean (Glycine max L. Merr. cv Bragg)-Rhizobium japonicum (strain 3I1b110) symbioses. Three different techniques, acetylene reduction, (15)N(2) fixation and relative abundance of ureides ([ureides/(ureides + nitrate + alpha-amino nitrogen)] x 100) in xylem exudate, gave similar results for the effect of nitrate on N(2) fixation by nodulated roots. After 2 days of treatment with 10 millimolar nitrate, acetylene reduction by nodulated roots was inhibited by 48% but there was no effect on either acetylene reduction by isolated bacteroids or in vitro activity of nodule cytoplasmic glutamine synthetase, glutamine oxoglutarate aminotransferase, xanthine dehydrogenase, uricase, or allantoinase. After 7 days, acetylene reduction by isolated bacteroids was almost completely inhibited but, except for glutamine oxoglutarate aminotransferase, there was still no effect on the nodule cytoplasmic enzymes. It was concluded that, when nitrate is supplied to an established symbiosis, inhibition of nodulated root N(2) fixation precedes the loss of the potential of bacteroids to fix N(2). This in turn precedes the loss of the potential of nodules to assimilate fixed N(2).

Regulation of Respiration in the Leaves and Roots of Two Lolium perenne Populations with Contrasting Mature Leaf Respiration Rates and Crop Yields

Measurements of O(2) uptake were made on leaves and roots of two populations of Lolium perenne L. cv S23 (GL66 and GL72), previously shown to have contrasting rates of CO(2) evolution and yields of dry matter. O(2) uptake was faster in the mature leaves of GL66 than those of GL72, but no difference was observed in the respiratory rates of meristematic leaf bases or mature roots. The growth rate of GL72 was faster than that of GL66. Cyanide resistance was substantial in mature leaves but the alternative path did not contribute to O(2) uptake in the dark. In both populations, adding malate and glycine stimulated O(2) uptake, but exogenous sucrose only stimulated when uncoupler was also present. The difference between the respiratory rates of the two populations was maintained under all investigated conditions. We conclude that the rate of mature leaf respiration in the dark in L. perenne is limited by adenylate control of glycolysis. The difference between the fast (GL66) and slow (GL72) respiring populations reflected a greater respiratory capacity and higher turnover of ATP in GL66. Alternative path capacity was also high in the roots of both and contributed substantially to O(2) uptake, as indicated by inhibition by salicylhydroxamic acid in the absence of KCN. The alternative path capacity of meristematic leaf bases was considerably less than that in mature leaves.Transverse and cross-sections were made of mature leaves of both populations to study anatomical features which might explain the differences in ATP turnover, suggested by the biochemical experiments. Leaves of GL72 were thicker but did not show a different anatomy when compared with GL66. The increased thickness was not due to more or larger cells but entirely to a larger intercellular volume.

Transport of NAD in Percoll-Purified Potato Tuber Mitochondria: Inhibition of NAD Influx and Efflux by N-4-Azido-2-nitrophenyl-4-aminobutyryl-3'-NAD

A mechanism by which intact potato (Solanum tuberosum) mitochondria may regulate the matrix NAD content was studied in vitro. If mitochondria were incubated with NAD(+) at 25 degrees C in 0.3 molar mannitol, 10 millimolar phosphate buffer (pH 7.4), 5 millimolar MgCl(2), and 5 millimolar alpha-ketoglutarate, the NAD pool size increased with time. In the presence of uncouplers, net uptake was not only inhibited, but NAD(+) efflux was observed instead. Furthermore, the rate of NAD(+) accumulation in the matrix space was strongly inhibited by the analog N-4-azido-2-nitrophenyl-4-aminobutyryl-3'-NAD(+). When suspended in a medium that avoided rupture of the outer membrane, intact purified mitochondria progressively lost their NAD(+) content. This led to a slow decrease of NAD(+)-linked substrates oxidation by isolated mitochondria The rate of NAD(+) efflux from the matrix space was strongly temperature dependent and was inhibited by the analog inhibitor of NAD(+) transport indicating that a carrier was required for net flux in either direction. It is proposed that uptake and efflux operate to regulate the total matrix NAD pool size.

Activation of NAD-linked malic enzyme in intact plant mitochondria by exogenous coenzyme A

O2 uptake by potato and cauliflower bud mitochondria oxidizing malate was progressively inhibited as the pH of the external medium was increased, in response to accumulation of oxaloacetate. Adding 0.5 mM coenzyme A to the medium reversed this trend by stimulating intramitochondrial NAD-linked malic enzyme at alkaline pH. In intact potato mitochondria, coenzyme A stimulation of malic enzyme was not observed when the external pH was above 7.5; in cauliflower mitochondria, coenzyme A stimulated even at pH 8. This difference in the response of intact mitochondria was attributed to an inherent difference in the properties of malic enzyme from the two tissues. Malic enzyme solubilized from potato mitochondria was inactive at pH values above 7.8, while that from cauliflower mitochondria retained its activity at pH 8 in the presence of coenzyme A. In potato mitochondria, coenzyme A stimulation of O2 uptake at alkaline pH was only observed when NAD+ was also provided exogenously. The results show that coenzyme A can be taken up by intact mitochondria and that pH, NAD+, and coenzyme A levels in the matrix act together to regulate malate oxidation.

Transport of coenzyme A in plant mitochondria

Oxoglutarate oxidation by purified potato mitochondria which had been stored at low temperature for 48 h or longer was stimulated by added coenzyme A. Exogenous coenzyme A was accumulated by potato mitochondria, both freshly prepared and aged, in a manner sensitive to uncouplers and low temperature. Coenzyme A was concentrated approximately 10-fold in the matrix under steady-state conditions. This coenzyme A uptake followed saturation kinetics with an apparent Km of 0.2 mM and a V of 4-6.5 nmol min-1 mg-1 protein, suggesting carrier-mediated transport. This transport was insensitive to an inhibitor of NAD+ transport. It is suggested that plant mitochondria possess a specific carrier for the net accumulation of coenzyme A.

Investigations of the role of the main light-harvesting chlorophyll-protein complex in thylakoid membranes. Reconstitution of depleted membranes from intermittent-light-grown plants with the isolated complex

The functions of the light-harvesting complex of photosystem II (LHC-II) have been studied using thylakoids from intermittent-light-grown (IML) plants, which are deficient in this complex. These chloroplasts have no grana stacks and only limited lamellar appression in situ. In vitro the thylakoids showed limited but significant Mg2+-induced membrane appression and a clear segregation of membrane particles into such regions. This observation, together with the immunological detection of small quantities of LHC-II apoproteins, suggests that the molecular mechanism of appression may be similar to the more extensive thylakoid stacking seen in normal chloroplasts and involve LHC-II polypeptides directly. To study LHC-II function directly, a sonication-freeze-thaw procedure was developed for controlled insertion of purified LHC-II into IML membranes. Incorporation was demonstrated by density gradient centrifugation, antibody agglutination tests, and freeze-fracture electron microscopy. The reconstituted membranes, unlike the parent IML membranes, exhibited both extensive membrane appression and increased room temperature fluorescence in the presence of cations, and a decreased photosystem I activity at low light intensity. These membranes thus mimic normal chloroplasts in this regard, suggesting that the incorporated LHC-II interacts with photosystem II centers in IML membranes and exerts a direct role in the regulation of excitation energy distribution between the two photosystems.