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Periplasmic Sud protein encoded by the Wolinella succinogenes catalyses the transfer of bound polysulfide-sulfur to the active site of the membrane bound polysulfide reductase. The homodimeric protein consists of 131 residues per monomer, each with one cysteine residue in the active site. Polysulfide-sulfur is covalently bound to the catalytic Cys residues of the Sud protein. In order to understand the structure-function relationship of this protein, the features of its solution structure determined by heteronuclear multidimensional NMR techniques are reported here. The first step of structure determination leads to resonance assignments using 15N/13C/2H- and 15N/13C-labeled protein. The sequential backbone and side chain resonance assignments have been successfully completed. Structure calculations were carried out using the ARIA program package. The structure is based on 2688 NOE-derived distance restraints, 68 backbone hydrogen bond restraints derived from 34 slow-exchanging backbone amide protons and 334 torsion angle restraints obtained from the TALOS program as well as 158 residual dipolar coupling restraints for the refinement of relative vector orientations. The three-dimensional structure of the Sud protein was determined with an averaged rootmean- square deviation of 0.72 Å and 1.28 Å for the backbone and heavy atoms, respectively, excluding the terminal residues. Without the poorly defined segment between residues 90-94 the average r.m.s.d. value drops down to 0.6 Å and 1.14 Å. The ensemble refined with residual dipolar coupling (rdc) restraints shows good convergence. The r.m.s.d. value for the backbone heavy atoms, excluding residues 90- 94, drops down from 0.97 to 0.66 for the rdc-refined ensemble. The relative orientation of the two monomers in the protein structures refined with residual dipolar coupling restraints are also different from those without residual dipolar coupling restraints. The structure determination of the dimeric protein has been hampered by the high molecular mass (30 kDa), severe peak degeneracy, and by the small number of experimental intermonomer NOEs (relative orientation problem of two monomers). For the resonance assignments of aliphatic side chain, many resonances were ambiguously assigned because of severe overlap of signals. The Sud dimer protein contains 17 Lys, 14 Leu and one His tag for each monomer. It complicated the resonance assignments. The conventional 3D 15N-separated TOCSY HSQC experiment failed because of the large molecular weight which results in line broadening and hence made the resonance assignments of side chains more difficult. The determined structure contains a five-stranded parallel ß-sheet enclosing a hydrophobic core, a two-stranded anti-parallel ß-sheet and seven a-helices. The dimer structure is stabilized predominantly by hydrophobic residues. Sud catalyses the transfer of the polysulfide-sulfur to cyanide, similar to rhodanese encoded by Azotobacter vinelandii (Bordo et al., 2000). The two proteins are similar in the active site environment primarily owing to the main-chain conformation of the active-site loop with the cysteine residue and with respect to the surrounding positively charged residues. The active-site loop (residues 89-95) in the Sud protein appears to be flexible, reflected by few assigned proton resonances of residues 90-94 in the active site. Despite their similarity in function and their similar structure in active site, the amino acid sequences and the folds of the two proteins are remarkably different. The negatively charged polysulfide interacts with positively charged R46, R67, and R94 and hence may be stabilized in structure. The mutation of one of the three arginines that are also conserved in rhodanese from A. vinelandii leads to a loss of sulfur-transfer activity. The polysulfide chain extends from inside of Sud protein to outside, where Sud may form contacts with polysulfide reductase. These contacts provide the possible polysulfide-sulfur transfer from Sud protein to the active site of polysulfide reductase.
One of the most species-rich ant-plant mutualisms worldwide is the palaeotropical Crematogaster-Macaranga system. The pioneer-tree genus Macaranga (Euphorbiaceae) is mainly inhabited by at least nine specific species of Crematogaster (Myrmicinae), of which eight belong to the subgenus Decacrema, as well as several species of Camponotus (Formicinae). Ant species are not randomly distributed among the Macaranga host plants but distinct patterns of associations have been found (Fiala et al., 1999 and references cited therein). The specificity of the associations is maintained in spite of common sympatric distribution of several host-plant species. Associations are, however, usually not species-specific and especially the Decacrema ants, that are the focus of this study, usually colonize several host plant species each. In this study I used a combined approach of ecological data as well as phylogenetic data based on mitochondrial DNA sequences in order to elucidate the factors determining the patterns found in the associations and the evolution of this mutualistic system between the specific Decacrema ant partners and their Macaranga host plants. Life history traits of seven different morphospecies found on the most common Macaranga host plants were compared and colony development was followed from colony founding on saplings to adult trees. Temporal variability of the associations between Decacrema ants and their respective host plants was also examined. Associations between Crematogaster ants of the subgenus Decacrema and their Macaranga host plants were found to be stable over periods of time, long enough to enable reproduction of the ant colony and (in most cases) the host plants, too. Life-expectancy of the ant colony seems to be shorter than that of the host plant in general. All adult trees still provide nesting space as well as food for the ants. Colonies from different morphospecies differed in longevity, the onset of alate production, queen number and mode of colony founding. The examined Decacrema species could be placed into two groups according to their life-history traits as well as on morphological grounds: The decamera-group and the captiosa-group, each named after one species that could be synonymized with one morphospecies included in the group. Members of the captiosa-group have larger colonies, presumably with a longer life-span, and a later onset of reproduction compared to the decamera-group. Additionally, queens of the captiosa-group found colonies on saplings as well as in the crown region of bigger trees, whereas queens of the decamera-group found colonies on saplings and small treelets only. Queens belonging to the captiosa-group are brown with relatively large eyes (= 1/3 of the head length), whereas queens from the decamera-group are smaller in size, are dark brown to black in colour and have smaller eyes (< 1/3 of the head length). On some of the host plants examined in this study lifespan of the host plant and their specific ant partners seemed to be well matched whereas on others an ontogenetic succession of specific Decacrema partner ants was found, when host plants were abandoned due to the death of comparatively short-lived ant colonies, usually from species belonging to the decamera-group. Ant-partners of saplings or young plants often differed from specific partner ants found on bigger trees. Only species belonging to the captiosa-group were found to re-colonize the crown region of adult trees, thus facilitating a change of ant species, when longlived host plant species were colonized by relatively short-lived species from the decamera-group first. When long -lived host plants were colonized by long-lived species from the captiosa-group associations were stabler: I did not find any temporal variation in ant-inhabitants then. Life-span of the ant colony as well colony founding behaviour of the different partner ant species therefore play an important role for these ontogenetic changes and the specificity of the associations over time. For the host plant the ontogenetic changes have a strong impact as uninhabited host plants that are not patrolled by workers of specific ant partners suffer higher herbivore damage. Uninhabited host plants may also be colonized by unspecific arboreal ants that only make use of the nesting space and/ or food offered by the plant but do not confer protection against herbivores. Stable associations with a specific ant partner are therefore most beneficial for the host plants. Usually ant colonies are monogynous, but changes in the colony structure were found locally in two Decacrema species. I found colonies that turned secondarily polygynous, possibly after the death of the original founding queen. Secondary polygyny therefore can prolong the life-span of the antcolony on its host plant, leading to a parallel life-history and stable association as it was the case in Macaranga bancana-Crematogaster captiosa. However, in the other association (Macaranga hypoleuca-Crematogaster cf. decamera) life-expectancy of the ant-colony is still much shorter than that of its host plant species, leading to a change in the specific ant partner at a later stage. Pleometrotic foundress associations that directly led to polygynous colonies in one species were also found locally, a phenomenon hardly ever reported from ants in general. Foundress associations were found to be more successful in establishing colonies than single queens. I found indications that this change in colony founding behaviour might be due to interspecific competition for the same host plant species with another Decacrema species specific to Macaranga. For the phylogenetic analysis partial mitochondrial cytochrome oxidase I and II were sequenced and Neighbor-Joining, Maximum Parsimony, Maximum Likelihood as well as Bayesian analyses were performed. The four different analyses yielded phenetic as well as phylogenetic trees that all had a similar topology. Ants of the subgenus Decacrema formed a monophyletic clade, indicating a single colonization event at the beginning of the Macaranga-Decacrema symbiotic system. In the phylogenetic analysis the decamera-group as well as the captiosa-group were confirmed and clearly separated from each other. However, two species that would have been placed into the decamera-group, due to morphological as well as life-history traits, formed a third separate clade within the Decacrema. These two species (msp. 7- group) as well as the decamera-group came out as the basal groups in the phylogenetic analysis. Thus, life -history traits of these two groups (relatively small colonies, early onset of alate production, colony founding in ground region only) would be the ancestral state for Macarangaassociated ants of the subgenus Decacrema. Changes in colony structure, like secondary polygyny, were found in the captiosa- as well as the decamera-group and are therefore independent of the affiliation within the phylogeny. I did not find evidence for strict cocladogenesis between the subgenus Decacrema and their Macaranga host-plants, although ecological interactions between the two partner groups are close and associations can be rather specific. The phylogenies presented here, along with the known association patterns indicate that host-shifting of the ants is common in some of the species, opening the possibility of sympatric speciation as a result of increased host usage. Additionally, the considerable geographic substructuring found in the phylogenetic trees suggests that allopatric speciation has played a major role in diversification of the Decacrema ants.
The light-harvesting chlorophyll a/b protein complex (LHC-II) is the major collector of solar energy in all plants and it binds about half of the chlorophyll in green plants. LHCII is a trimer in the photosynthetic membrane; each monomer consists of 232 amino acids, binds and orients a minimum of 12 chlorophyll molecules and three caroteinoids (two luteins and one neoxanthin) for light-harvesting and energy transfer. Although, the structure of LHC-II has been determined at 3.4 Å resolution by electron microscopy of two-dimensional crystals (Kühlbrandt et al., 1994), this is not sufficient to allow a complete understanding of the mechanism of energy transfer from LHC-II to the reaction centre, since the effective resolution in the z dimension is 4.9 Å. In fact, the chemical difference between Chl a and Chl b, which has a formyl group instead of the methyl group at the 7-position in the chlorin ring, is too small to be detected at this level of resolution. In addition, the orientation of the chlorophyll tetrapyrroles have not been determined unambiguously. This information is essential for a detailed understanding of the energy transfer within the complex and to the reaction centres of photosystem II and I (PSII and PSI). X-ray crystallography of three dimensional (3D) crystals may yield a more complete structure at high resolution. 3D crystals have been grown from LHC-II isolated from pea leaves using a standard purification procedure (Burke et al., 1978). The thylakoid membranes are solubilised in Triton X-100 and further purified by sucrose gradient ultra centrifugation. The LHC-II fraction is salt precipitated and pellets resuspended at the chlorophyll a/b ratio 2.8 mg/ml in 0.9 % Nonyl-glucoside. Crystals are currently obtained by vapour diffusion in hanging drops. These crystals are thin hexagonal plates, have a fairly large unit cell and diffract quite weakly. The high level of the background is due both to the detergent, necessary for protein solubilisation, and lipids, required for the trimer and crystals formation. However, three data sets, each from one single crystal have been collected up to 3.2 Å resolution over a rotation range of 135°. The crystals were exposed to a very highly collimated and brilliant beam (ID-14 EH1 at ESRF, Grenoble, France) and were kept under a stream of cold nitrogen to prevent radiation damage. Data were successfully integrated using the program XDS by Kabsch (1993). The crystals were found to belong to the space group P6 22 3 and have unit cell dimensions of a=128.45, b=128.45, c=135.32, a= ß=90º, ?=120. The solution of the phase problem was tackled by molecular replacement using, as a search model, the LHC-II structure solved by electron cryo-microscopy studies of twodimensional crystals (Kühlbrandt et al. 1994). Three different programs were tested: the most used AMoRe (Navaza et al., 1994) and the brute force based program Brute (Fujinaga