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SUMMARY Introducing a carbon concentrating mechanism and a faster Rubisco from cyanobacteria into higher plant chloroplasts could improve photosynthetic performance by increasing the rate of CO2 fixation while decreasing losses caused by photorespiration. We previously demonstrated that tobacco plants will grow photoautotrophically using Synechococcus elongatus Rubisco, although the plants exhibited considerably slower growth than wild-type and required supplementary CO2. Because of concerns that vascular plant assembly factors might not be adequate for assembly of a cyanobacterial Rubisco, prior transgenic plants included the cyanobacterial chaperone RbcX or the carboxysomal protein CcmM35. Here we show that neither RbcX nor CcmM35 is needed for assembly of active cyanobacterial Rubisco. Furthermore, by altering the gene regulatory sequences on the Rubisco transgenes, cyanobacterial Rubisco expression was enhanced and the transgenic plants grew at near wild-type growth rates, though still requiring elevated CO2. We performed detailed kinetic characterization of the enzymes produced with and without the RbcX and CcmM35 cyanobacterial proteins. These transgenic plants exhibit photosynthetic characteristics that confirm the predicted benefits of non-native forms of Rubisco with higher carboxylation rate constants in vascular plants and the potential nitrogen use efficiency that may be gained provided that adequate CO2 can be concentrated near the enzyme.

SUMMARY Photosynthetic efficiency of C3 plants suffers from the reaction of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) with O2 instead of CO2, leading to the costly process of photorespiration. Increasing the concentration of CO2 around Rubisco is a strategy used by photosynthetic prokaryotes such as cyanobacteria for more efficient incorporation of inorganic carbon. Engineering the cyanobacterial CO2 concentrating mechanism, the carboxysome, into chloroplasts is an approach to enhance photosynthesis or to compartmentalize other biochemical reactions to confer new capabilities on transgenic plants. We have chosen to explore the possibility of producing β-carboxysomes from Synechococcus elongatus PCC7942, a model freshwater cyanobacterium. Using the agroinfiltration technique, we have transiently expressed multiple β-carboxysomal proteins (CcmK2, CcmM, CcmL, CcmO and CcmN) in Nicotiana benthamiana with fusions that target these proteins into chloroplasts and that provide fluorescent labels for visualizing the resultant structures. By confocal and electron microscopic analysis, we have observed that the shell proteins of the β-carboxysome are able to assemble in plant chloroplasts into highly organized assemblies resembling empty microcompartments. We demonstrate that a foreign protein can be targeted with a 17-amino-acid CcmN peptide to the shell proteins inside chloroplasts. Our experiments establish the feasibility of introducing carboxysomes into chloroplasts for potential compartmentalization of Rubisco or other proteins.

Summary Cytochrome bo3 is the major respiratory oxidase located in the cytoplasmic membrane of E. coli when grown under high oxygen tension. The enzyme catalyzes the 2-electron oxidation of ubiquinol-8 and the 4-electron reduction of dioxygen to water. When solubilized and isolated using dodecylmaltoside, the enzyme contains one equivalent of ubiquinone-8, bound at a high affinity site (QH). The quinone bound at the QH site can form a stable semiquinone, and the amino acid residues which hydrogen bond to the semiquinone have been identified. In the current work, it is shown that the tightly bound ubiquinone-8 at the QH site is not displaced by ubiquinol-1 even during enzyme turnover. Furthermore, the presence of high affinity inhibitors, HQNO and aurachin C1-10, do not displace ubiquinone-8 from the QH site. The data clearly support the existence of a second binding site for ubiquinone, the QL site, which can rapidly exchange with the substrate pool. HQNO is shown to bind to a single site on the enzyme and to prevent formation of the stable ubisemiquinone, though without displacing the bound quinone. Inhibition of the steady state kinetics of the enzyme indicates that aurachin C1-10 may compete for binding with quinol at the QL site while, at the same time, preventing formation of the ubisemiquinone at the QH site. It is suggested that the two quinone binding sites may be adjacent to each other or partially overlap.

The selective 15 N isotope labeling was used for the identification of the nitrogen involved in an hydrogen bond formation with the semiquinone in the high-affinity Q H site in the cytochrome bo 3 ubiquinol oxidase. This nitrogen produces dominating contribution to X-Band 14 N ESEEM spectra. The 2D ESEEM (HYSCORE) experiments with the Q H site SQ in the series of selectively 15 N labeled bo 3 oxidase proteins have directly identified the N ε of R71 as an H-bond donor. In addition, selective 15 N labeling has allowed us for the first time to determine weak hyperfine couplings with the side-chain nitrogens from all residues around the SQ. Those are reflecting a distribution of the unpaired spin density over the protein in the SQ state of the quinone processing site.E. coli cytochrome (cyt) bo 3 ubiquinol oxidase catalyzes the two-electron oxidation of ubiquinol and the four-electron reduction of O 2 to water. The enzyme contains three redoxactive metal centers: a low spin heme b, which is involved in quinol oxidation, and the heme o 3 /Cu B bimetallic center, which is the site where O 2 binds and is reduced to water. The ubiquinol oxidation occurs with a semiquinone (SQ) intermediate in an overall reaction that releases two protons to the periplasm. The enzyme contains two Q sites 1-6 : a low affinity site (Q L ), which is equilibrated with the quinone pool in the membrane and functions as the substrate (QH 2 ) binding site, and a high affinity (Q H ) site, from which Q is not readily removed, and which stabilizes a SQ. 7-10 The Q H site quinone appears to function as a tightly bound cofactor, similar to the Q A site of the reaction centers. The X-ray structure of cyt bo 3 11 does not contain any bound quinone, but mutational substitutions of R71, D75, H98, and Q101Correspondence to: Robert B. Gennis; Sergei A. Dikanov, dikanov@uiuc.edu. The interaction of the SQ with the protein environment in cyt bo 3 has been studied by pulsed EPR spectroscopy. X-band ESEEM data show that there is one H-bond to the Q H SQ from a nitrogen donor. 5,6,12,13 The speculated identification of this nitrogen has been based on the quadrupole coupling constant (qcc) determined from the ESEEM spectra. 5,6,12,13 Its value, K=e 2 qQ/4h=0.93 MHz, most closely corresponds to the nitrogen from an NH or NH 2 group. 12,13 This value is ∼10% larger than the qcc for the peptide amide nitrogen and significantly exceeds the qcc of the protonated nitrogens in histidine. Hence, the most likely candidates for the H-bond donor are the nitrogens from the side chains of R71 or Q101, though a peptide backbone nitrogen cannot be ruled out completely. NIH Public AccessTo overcome the existing uncertainties and to identify directly the H-bonded nitrogen, we employed 15 N selective labeling in different residues. Proteins were labeled as follows: 1) 15 N uniformly labeled Arg; 2) 15 N uniformly labeled His; 3) Gln with 15 N only in the N ε position; 4) Arg with 15 N only in the two N η positions; 5) Arg with 15 N only in the peptide nitrogen (Scheme 1)...

Selective 15N isotope labeling of the cytochrome bo3 ubiquinol oxidase from E. coli with auxotrophs was used to characterize the hyperfine couplings with the side-chain nitrogens from R71, H98, and Q101 residues and peptide nitrogens from R71 and H98 residues around the semiquinone (SQ) at the high-affinity QH site. The 2D ESEEM (HYSCORE) data have directly identified the Nε of R71 as an H-bond donor carrying the largest amount of the unpaired spin density. In addition, weaker hyperfine couplings with the side-chain nitrogens from all residues around the SQ were determined. These hyperfine couplings reflect a distribution of the unpaired spin density over the protein in the SQ state of the QH site and strength of interaction with different residues. The approach was extended to the virtually inactive D75H mutant, where the intermediate SQ is also stabilized. We found that the Nε from a histidine residue, presumably H75, carries most of the unpaired spin density instead of the Nε of R71, as in the wild-type bo3. However, the detailed characterization of the weakly coupled 15Ns from selective labeling of R71 and Q101 in D75H was precluded by overlap of the 15N lines with the much stronger ~1.6 MHz line from quadrupole triplet of the strongly coupled 14Nε from H75. Therefore, a reverse labeling approach, in which the enzyme was uniformly labeled except for selected amino acid types, was applied in order to probe the contribution of R71 and Q101 to the 15N signals. Such labeling has shown only weak coupling with all nitrogens of R71 and Q101. We utilize density functional theory based calculations to model the available information about 1H, 15N and 13C hyperfine couplings for the QH site and to describe the protein-substrate interactions in both enzymes. In particular, we identify the factors responsible for the asymmetric distribution of the unpaired spin density and ponder the significance of this asymmetry to the quinone’s electron transfer function.

Amino-acid selective isotope labeling of proteins offers numerous advantages in mechanistic studies by revealing structural and functional information unattainable from a crystallographic approach. However, efficient labeling of proteins with selected amino acids necessitates auxotrophic hosts, which are often not available. We have constructed a set of auxotrophs in a commonly used Escherichia coli expression strain C43(DE3), a derivative of E. coli BL21(DE3), which can be used for isotopic labeling of individual amino acids or sets of amino acids. These strains have general applicability to either soluble or membrane proteins that can be expressed in E. coli. We present examples in which proteins are selectively labeled with 13C- and 15N-amino acids and studied using magic-angle spinning solid-state NMR and pulsed EPR, demonstrating the utility of these strains for biophysical characterization of membrane proteins, radical-generating enzymes and metalloproteins.

For screening a pool of potential substrates that load carrier domains found in non-ribosomal peptide synthetases, large molecule mass spectrometry is shown to be an ideal, unbiased assay. Combining the high resolving power of Fourier-Transform Mass Spectrometry with the ability of adenylation domains to select their own substrates, the mass change that takes place upon formation of a covalent intermediate thus identifies the substrate. This assay has an advantage over traditional radiochemical assays in that many substrates, the substrate pool, can be screened simultaneously. Using proteins on the nikkomycin, clorobiocin, coumermycin A 1 , yersiniabactin, pyochelin and enterobactin biosynthetic pathways as proof-of-principle, preferred substrates are readily identified from substrate pools. Furthermore this assay can be used to provide insight into the timing of tailoring events of biosynthetic pathways as demonstrated using the bromination reaction found on the jamaicamide biosynthetic pathway. Finally, this assay can provide insight into the role and function of orphan gene clusters for which the encoded natural product is unknown. This is demonstrated by identifying the substrates for two NRPS modules from the genes pksN and pksJ, that are found on an orphan NRPS/PKS hybrid cluster from Bacillus subtilis. This new assay format is especially timely for activity screening in an era when new types of thiotemplate assembly lines that defy classification are being discovered at an accelerating rate. KeywordsElectrospray Fourier transform mass spectrometry; Non-ribosomal peptide synthetase; Polyketide synthase; Carrier domains; Post-translational modification Almost weekly, new non-ribosomal peptide synthetase (NRPS) gene clusters encoding for proteins that biosynthesize bioactive compounds are discovered (1-4). Since many of the compounds produced by non-ribosomal peptide synthetase (NRPS) as well as polyketide * To whom correspondence should be addressed, kelleher@scs.uiuc.edu, fax number 217 244-8068 and phone number 217 NIH Public Access Author ManuscriptBiochemistry. Author manuscript; available in PMC 2008 October 9. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript synthase (PKS) paradigm have potent medicinal utility or are involved in virulence, and display unusual chemistry, they are of great academic and industrial interest. Therefore we wanted to develop an alternative method for substrate screening to complement the more traditional radioactive assays (5,6). In NRPS and PKS systems, the substrates and intermediates are loaded onto and processed while attached to the pantetheinyl functionality on a carrier domain (7,8).Examples of NRPS or hybrid NRPS/PKS natural products for which substrates load onto carrier domains are: the siderophore pyoverdine (9), a virulence factor excreted by pseudomonads, the antimicrobial agents penicillin (10), vancomycin (11) gramicidin (12) and the antitumor agent calicheamycin (13). With ∼300 genomes sequenced and available in the public ...