Two Unrelated 8-Vinyl Reductases Ensure Production of Mature Chlorophylls in Acaryochloris marina

Abstract

The major photopigment of the cyanobacterium Acaryochloris marina is chlorophyll d, while its direct biosynthetic precursor, chlorophyll a, is also present in the cell. These pigments, along with the majority of chlorophylls utilized by oxygenic phototrophs, carry an ethyl group at the C-8 position of the molecule, having undergone reduction of a vinyl group during biosynthesis. Two unrelated classes of 8-vinyl reductase involved in the biosynthesis of chlorophylls are known to exist, BciA and BciB. The genome of Acaryochloris marina contains open reading frames (ORFs) encoding proteins displaying high sequence similarity to BciA or BciB, although they are annotated as genes involved in transcriptional control (nmrA) and methanogenesis (frhB), respectively. These genes were introduced into an 8-vinyl chlorophyll a-producing ΔbciB strain of Synechocystis sp. strain PCC 6803, and both were shown to restore synthesis of the pigment with an ethyl group at C-8, demonstrating their activities as 8-vinyl reductases. We propose that nmrA and frhB be reassigned as bciA and bciB, respectively; transcript and proteomic analysis of Acaryochloris marina reveal that both bciA and bciB are expressed and their encoded proteins are present in the cell, possibly in order to ensure that all synthesized chlorophyll pigment carries an ethyl group at C-8. Potential reasons for the presence of two 8-vinyl reductases in this strain, which is unique for cyanobacteria, are discussed.IMPORTANCE The cyanobacterium Acaryochloris marina is the best-studied phototrophic organism that uses chlorophyll d for photosynthesis. Unique among cyanobacteria sequenced to date, its genome contains ORFs encoding two unrelated enzymes that catalyze the reduction of the C-8 vinyl group of a precursor molecule to an ethyl group. Carrying a reduced C-8 group may be of particular importance to organisms containing chlorophyll d. Plant genomes also contain orthologs of both of these genes; thus, the bacterial progenitor of the chloroplast may also have contained both bciA and bciB.

Engineering of B800 bacteriochlorophyll binding site specificity in the Rhodobacter sphaeroides LH2 antenna

“…Acaryochloris marina cells were grown as described for Synechocystis sp. PCC 6803 with the addition of 32 g L −1 instant ocean (Aquarium systems) [ 63 ]. Fischerella thermalis PCC 7521 cells were grown as described in [ 64 ].…”

Biosynthesis of the modified tetrapyrroles—the pigments of life

“…Although plant and green algal genomes contain homologs of both bciA and bciB, most cyanobacterial genomes only contain homologs of one gene, either bciA or bciB. An exception is found in the marine cyanobacterium Acaryochloris marina, which has both forms of divinyl reductase (229). Whereas BciA uses NADPH (223) as the reductant, BciB from GSB contains an FAD cofactor and [4Fe4S] clusters and uses ferredoxin as the reductant for this reaction (228).…”

Type IV Pili-Independent Photocurrent Production by the Cyanobacterium Synechocystis sp. PCC 6803

“…End-point reverse transcriptase PCR analysis of Synechocystis strains was performed as described previously for Acaryochloris marina (Chen et al, 2016). Briefly, Synechocystis cells were harvested at mid-log phase (OD 750 = ∼0.6), and total RNA was isolated by the hot TRIzol method (Pinto et al, 2009).…”