Abstract
Green bacteria are chlorophotorophs that synthesize bacteriochlorophyll (BChl) c, d, or e, which assemble into supramolecular, nanotubular structures in large light-harvesting structures called chlorosomes. The biosynthetic pathways of these chlorophylls are known except for one reaction. Null mutants of bciD, which encodes a putative radical S-adenosyl-l-methionine (SAM) protein, are unable to synthesize BChl e but accumulate BChl c; however, it is unknown whether BciD is sufficient to convert BChl c (or its precursor, bacteriochlorophyllide (BChlide) c) into BChl e (or BChlide e). To determine the function of BciD, we expressed the bciD gene of Chlorobaculum limnaeum strain DSMZ 1677T in Escherichia coli and purified the enzyme under anoxic conditions. Electron paramagnetic resonance spectroscopy of BciD indicated that it contains a single [4Fe-4S] cluster. In assays containing SAM, BChlide c or d, and sodium dithionite, BciD catalyzed the conversion of SAM into 5'-deoxyadenosine and BChlide c or d into BChlide e or f, respectively. Our analyses also identified intermediates that are proposed to be 71-OH-BChlide c and d Thus, BciD is a radical SAM enzyme that converts the methyl group of BChlide c or d into the formyl group of BChlide e or f This probably occurs by a mechanism involving consecutive hydroxylation reactions of the C-7 methyl group to form a geminal diol intermediate, which spontaneously dehydrates to produce the final products, BChlide e or BChlide f The demonstration that BciD is sufficient to catalyze the conversion of BChlide c into BChlide e completes the biosynthetic pathways for all "Chlorobium chlorophylls."
Highlights
Chlorobium luteolum DSM 273T has two bchK genes, one of which is similar to the bchK gene of C. tepidum and one of which belongs to the clade of enzymes that are found in brown-colored, BChl e-producing GSB strains
The presence of a bchK2 gene, the product of which can presumably esterify BChlide e with farnesol, might “predispose” Chlorobium luteolum to transformation from a BChl c-producing into a BChl e-producing strain by the acquisition of bciD, the horizontally transferred, conserved gene cluster does not include a gene for a chlorophyll synthase
These ideas can be tested in future studies by inactivating the bchK1 and bchK2 genes in C. limnaeum as well as by introducing the bciD gene into C. tepidum in combination with each of the two bchK genes encoding the two paralogous esterification enzymes of C. limnaeum
Summary
Green bacteria are chlorophotorophs that synthesize bacteriochlorophyll (BChl) c, d, or e, which assemble into supramolecular, nanotubular structures in large light-harvesting structures called chlorosomes. BciD is a radical SAM enzyme that converts the methyl group of BChlide c or d into the formyl group of BChlide e or f This probably occurs by a mechanism involving consecutive hydroxylation reactions of the C-7 methyl group to form a geminal diol intermediate, which spontaneously dehydrates to produce the final products, BChlide e or BChlide f. Mutational analyses made possible by the availability of genome sequences for numerous green bacteria led to the establishment of the pathway for the synthesis of BChls c and d (12, 18 –25). These BChls are synthesized from a pathway that diverges from chlorophyllide (Chlide) a (Fig. 1) [18]. The methylation reactions catalyzed by BchU, BchQ, and BchR help to tune the absorption properties of BChl c, d, and e in chlorosomes and lead to inhomogeneous broadening of the nearinfrared absorption band of aggregated BChls in chlorosomes
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