Article

Structural Variability in Wild-Type and bchQ bchR Mutant Chlorosomes of the Green Sulfur Bacterium Chlorobaculum tepidum.

Leiden Institute of Chemistry, Leiden University , P.O. Box 9502, 2300 RA Leiden, The Netherlands.
Biochemistry (impact factor: 3.42). 05/2012; 51(22):4488-98. DOI:10.1021/bi201817x
Source: PubMed

ABSTRACT The self-aggregated state of bacteriochlorophyll (BChl) c molecules in chlorosomes belonging to a bchQ bchR mutant of the green sulfur bacteria Chlorobaculum tepidum, which mostly produces a single 17(2)-farnesyl-(R)-[8-ethyl,12-methyl]BChl c homologue, was characterized by solid-state nuclear magnetic resonance (NMR) spectroscopy and high-resolution electron microscopy. A nearly complete (1)H and (13)C chemical shift assignment was obtained from well-resolved homonuclear (13)C-(13)C and heteronuclear (1)H-(13)C NMR data sets collected from (13)C-enriched chlorosome preparations. Pronounced doubling (1:1) of specific (13)C and (1)H resonances revealed the presence of two distinct and nonequivalent BChl c components, attributed to all syn- and all anti-coordinated parallel stacks, depending on the rotation of the macrocycle with respect to the 3(1)-methyl group. Steric hindrance from the 20-methyl functionality induces structural differences between the syn and anti forms. A weak but significant and reproducible reflection at 1/0.69 nm(-1) in the direction perpendicular to the curvature of cylindrical segments observed with electron microscopy also suggests parallel stacking of BChl c molecules, though the observed lamellar spacing of 2.4 nm suggests weaker packing than for wild-type chlorosomes. We propose that relaxation of the pseudosymmetry observed for the wild type and a related BChl d mutant leads to extended domains of alternating syn and anti stacks in the bchQ bchR chlorosomes. Domains can be joined to form cylinders by helical syn-anti transition trajectories. The phase separation in domains on the cylindrical surface represents a basic mechanism for establishing suprastructural heterogeneity in an otherwise uniform supramolecular scaffolding framework that is well-ordered at the molecular level.

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  • Article: Bacteriochlorophyll f: properties of chlorosomes containing the "forbidden chlorophyll".
    Kajetan Vogl, Marcus Tank, Gregory S Orf, Robert E Blankenship, Donald A Bryant
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    ABSTRACT: The chlorosomes of green sulfur bacteria (GSB) are mainly assembled from one of three types of bacteriochlorophylls (BChls), BChls c, d, and e. By analogy to the relationship between BChl c and BChl d (20-desmethyl-BChl c), a fourth type of BChl, BChl f (20-desmethyl-BChl e), should exist but has not yet been observed in nature. The bchU gene (bacteriochlorophyllide C-20 methyltransferase) of the brown-colored green sulfur bacterium Chlorobaculum limnaeum was inactivated by conjugative transfer from Eshcerichia coli and homologous recombination of a suicide plasmid carrying a portion of the bchU. The resulting bchU mutant was greenish brown in color and synthesized BChl f(F). The chlorosomes of the bchU mutant had similar size and polypeptide composition as those of the wild type (WT), but the Q(y) absorption band of the BChl f aggregates was blue-shifted 16 nm (705 nm vs. 721 nm for the WT). Fluorescence spectroscopy showed that energy transfer to the baseplate was much less efficient in chlorosomes containing BChl f than in WT chlorosomes containing BChl e. When cells were grown at high irradiance with tungsten or fluorescent light, the WT and bchU mutant had identical growth rates. However, the WT grew about 40% faster than the bchU mutant at low irradiance (10 μmol photons m(-2) s(-1)). Less efficient energy transfer from BChl f aggregates to BChl a in the baseplate, the much slower growth of the strain producing BChl f relative to the WT, and competition from other phototrophs, may explain why BChl f is not observed naturally.
    Frontiers in microbiology. 01/2012; 3:298.
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Keywords

1)H resonances
 
13)C chemical shift assignment
 
13)C-enriched chlorosome preparations
 
20-methyl functionality induces structural differences
 
3(1)-methyl group
 
alternating syn
 
anti-coordinated parallel stacks
 
BChl c molecules
 
bchQ bchR chlorosomes
 
bchQ bchR mutant
 
cylindrical surface
 
green sulfur bacteria Chlorobaculum tepidum
 
helical syn-anti transition trajectories
 
nonequivalent BChl c components
 
related BChl d mutant
 
self-aggregated state
 
solid-state nuclear magnetic resonance
 
uniform supramolecular scaffolding framework
 
well-resolved homonuclear
 
wild-type chlorosomes