S-adenosyl-L-methionine:magnesium-protoporphyrin IX O-methyltransferase from Rhodobacter capsulatus: mechanistic insights and stimulation with phospholipids

Highly integrated adaptive mechanisms in Spiribacter halalkaliphilus, a bacterium abundant in Chinese soda-saline lakes

Soda-saline lakes are polyextreme environments inhabited by many haloalkaliphiles, including one of the most abundant Spiribacter species. However, its mechanisms of adaptation are not ecophysiologically characterized. Based on a large-scale cultivation strategy, we obtained a representative isolate of this Spiribacter species whose relative abundance was the highest (up to 15.63%) in a wide range of salinities in the soda-saline lakes in Inner Mongolia, China. This species is a chemoorganoheterotrophic haloalkaliphile. It has a small and streamlined genome and utilizes a wide variety of compatible solutes to resist osmotic pressure and multiple monovalent cation/proton antiporters for pH homeostasis. In addition to growth enhancement by light under microaerobic conditions, cell growth, organic substrate consumption and polyhydroxybutyrate biosynthesis were also improved by inorganic sulfide. Both quantitative RT-PCR and enzymatic assays verified that sulfide:quinone oxidoreductase was upregulated during this process. Metatranscriptomic analysis indicated that all genes related to environmental adaptation were transcribed in natural environments. Overall, this study has identified a novel abundant haloalkaliphile with multiple and highly integrated adaptive strategies and found that inorganic sulfide was able to improve the adaptation of a heterotroph to polyextreme environments.

A Thylakoid Membrane Protein Functions Synergistically with GUN5 in Chlorophyll Biosynthesis

Chlorophyll (Chl) is essential for photosynthetic reactions and chloroplast development. While the enzymatic pathway for Chl biosynthesis is well established, the regulatory mechanism underlying the homeostasis of Chl levels remains largely unknown. In this study, we identified CBD1 (Chlorophyll Biosynthetic Defect1), which functions in the regulation of chlorophyll biosynthesis. The CBD1 gene was expressed specifically in green tissues and its protein product was embedded in the thylakoid membrane. Furthermore, CBD1 was precisely co-expressed and functionally correlated with GUN5 (Genome Uncoupled 5). Analysis of chlorophyll metabolic intermediates indicated that cbd1 and cbd1gun5 mutants over-accumulated magnesium protoporphyrin IX (Mg-Proto IX). In addition, the cbd1 mutant thylakoid contained less Mg than the wild type not only as a result of lower Chl content, but also implicating CBD1 in Mg transport. This was supported by the finding that CBD1 complemented a Mg²⁺ uptake-deficient Salmonella strain under low Mg conditions. Taken together, these results indicate that CBD1 functions synergistically with CHLH/GUN5 in Mg-Proto IX processing, and may serve as a Mg-transport protein to maintain Mg homeostasis in the chloroplast.

Structure of the cyanobacterial Magnesium Chelatase H subunit determined by single particle reconstruction and small-angle X-ray scattering

The biosynthesis of chlorophyll, an essential cofactor for photosynthesis, requires the ATP-dependent insertion of Mg(2+) into protoporphyrin IX catalyzed by the multisubunit enzyme magnesium chelatase. This enzyme complex consists of the I subunit, an ATPase that forms a complex with the D subunit, and an H subunit that binds both the protoporphyrin substrate and the magnesium protoporphyrin product. In this study we used electron microscopy and small-angle x-ray scattering to investigate the structure of the magnesium chelatase H subunit, ChlH, from the thermophilic cyanobacterium Thermosynechococcus elongatus. Single particle reconstruction of negatively stained apo-ChlH and Chl-porphyrin proteins was used to reconstitute three-dimensional structures to a resolution of ∼30 Å. ChlH is a large, 148-kDa protein of 1326 residues, forming a cage-like assembly comprising the majority of the structure, attached to a globular N-terminal domain of ∼16 kDa by a narrow linker region. This N-terminal domain is adjacent to a 5 nm-diameter opening in the structure that allows access to a cavity. Small-angle x-ray scattering analysis of ChlH, performed on soluble, catalytically active ChlH, verifies the presence of two domains and their relative sizes. Our results provide a basis for the multiple regulatory and catalytic functions of ChlH of oxygenic photosynthetic organisms and for a chaperoning function that sequesters the enzyme-bound magnesium protoporphyrin product prior to its delivery to the next enzyme in the chlorophyll biosynthetic pathway, magnesium protoporphyrin methyltransferase.

BchJ and BchM interact in a 1 : 1 ratio with the magnesium chelatase BchH subunit of Rhodobacter capsulatus

Substrate channeling between the enzymatic steps in the (bacterio)chlorophyll biosynthetic pathway catalyzed by magnesium chelatase (BchI/ChlI, BchD/ChlD and BchH/ChlH subunits) and S-adenosyl-L-methionine:magnesium-protoporphyrin IX O-methyltransferase (BchM/ChlM) has been suggested. This involves delivery of magnesium-protoporphyrin IX from the BchH/ChlH subunit of magnesium chelatase to BchM/ChlM. Stimulation of BchM/ChlM activity by BchH/ChlH has previously been shown, and physical interaction of the two proteins has been demonstrated. In plants and cyanobacteria, there is an added layer of complexity, as Gun4 serves as a porphyrin (protoporphyrin IX and magnesium-protoporphyrin IX) carrier, but this protein does not exist in anoxygenic photosynthetic bacteria. BchJ may play a similar role to Gun4 in Rhodobacter, as it has no currently assigned function in the established pathway. Purified recombinant Rhodobacter capsulatus BchJ and BchM were found to cause a shift in the equilibrium amount of Mg-protoporphyrin IX formed in a magnesium chelatase assay. Analysis of this shift revealed that it was always in a 1 : 1 ratio with either of these proteins and the BchH subunit of the magnesium chelatase. The establishment of the new equilibrium was faster with BchM than with BchJ in a coupled magnesium chelatase assay. BchJ bound magnesium-protoporphyrin IX or formed a ternary complex with BchH and magnesium-protoporphyrin IX. These results suggest that BchJ may play a role as a general magnesium porphyrin carrier, similar to one of the roles of GUN4 in oxygenic organisms.

Biochemical characterization of two wheat phosphoethanolamine N-methyltransferase isoforms with different sensitivities to inhibition by phosphatidic acid

In plants the triple methylation of phosphoethanolamine to phosphocholine catalyzed by phosphoethanolamine N-methyltransferase (PEAMT) is considered a rate-limiting step in the de novo synthesis of phosphatidylcholine. Besides being a major membrane phospholipid, phosphatidylcholine can be hydrolyzed into choline and phosphatidic acid. Phosphatidic acid is widely recognized as a second messenger in stress signaling, and choline can be oxidized within the chloroplast to yield the putative osmoprotectant glycine betaine. Here we describe the cloning and biochemical characterization of a second wheat PEAMT isoform that has a four times higher specific activity than the previously described WPEAMT/TaPEAMT1 enzyme and is less sensitive to product inhibition by S-adenosyl homocysteine, but more sensitive to inhibition by phosphocholine. Both enzymes follow a sequential random Bi Bi mechanism and show mixed-type product inhibition patterns with partial inhibition for TaPEAMT1 and a strong non-competitive component for TaPEAMT2. An induction of TaPEAMT protein expression and activity is observed after cold exposure, ahead of an increase in gene expression. Our results demonstrate direct repression of in vitro enzymatic activities by phosphatidic acid for both enzymes, with TaPEAMT1 being more sensitive than TaPEAMT2 in the physiological concentration range. Other lipid ligands identified in protein-lipid overlays are phosphoinositide mono- as well as some di-phosphates and cardiolipin. These results provide new insights into the complex regulatory circuits of phospholipid biosynthesis in plants and underline the importance of head group biosynthesis in adaptive stress responses.

Kinetic analyses of the magnesium chelatase provide insights into the mechanism, structure, and formation of the complex

The metabolic pathway known as (bacterio)chlorophyll biosynthesis is initiated by magnesium chelatase (BchI, BchD, BchH). This first step involves insertion of magnesium into protoporphyrin IX (proto), a process requiring ATP hydrolysis. Structural information shows that the BchI and BchD subunits form a double hexameric enzyme complex, whereas BchH binds proto and can be purified as BchH-proto. We utilized the Rhodobacter capsulatus magnesium chelatase subunits using continuous magnesium chelatase assays and treated the BchD subunit as the enzyme with both BchI and BchH-proto as substrates. Michaelis-Menten kinetics was observed with the BchI subunit, whereas the BchH subunit exhibited sigmoidal kinetics (Hill coefficient of 1.85). The BchI.BchD complex had intrinsic ATPase activity, and addition of BchH greatly increased ATPase activity. This was concentration-dependent and gave sigmoidal kinetics, indicating there is more than one binding site for the BchH subunit on the BchI.BchD complex. ATPase activity was approximately 40-fold higher than magnesium chelatase activity and continued despite cessation of magnesium chelation, implying one or more secondary roles for ATP hydrolysis and possibly an as yet unknown switch required to terminate ATPase activity. One of the secondary roles for BchH-stimulated ATP hydrolysis by a BchI.BchD complex is priming of BchH to facilitate correct binding of proto to BchH in a form capable of participating in magnesium chelation. This porphyrin binding is the rate-limiting step in catalysis. These data suggest that ATP hydrolysis by the BchI.BchD complex causes a series of conformational changes in BchH to effect substrate binding, magnesium chelation, and product release.