Specific Gene bciD for C7-Methyl Oxidation in Bacteriochlorophyll e Biosynthesis of Brown-Colored Green Sulfur Bacteria

Jiro Harada,Masato Noguchi,Makio Yokono,Tadashi Mizoguchi,Yusuke Tsukatani,Souichirou Satoh,Hitoshi Tamiaki,Ayumi Tanaka,Nikolai Lebedev

doi:10.1371/journal.pone.0060026

Abstract

The gene named bciD, which encodes the enzyme involved in C7-formylation in bacteriochlorophyll e biosynthesis, was found and investigated by insertional inactivation in the brown-colored green sulfur bacterium Chlorobaculum limnaeum (previously called Chlorobium phaeobacteroides). The bciD mutant cells were green in color, and accumulated bacteriochlorophyll c homologs bearing the 7-methyl group, compared to C7-formylated BChl e homologs in the wild type. BChl-c homolog compositions in the mutant were further different from those in Chlorobaculum tepidum which originally produced BChl c: (31 S)-8-isobutyl-12-ethyl-BChl c was unusually predominant.

Highlights

Chlorophyll(Chl)s and bacteriochlorophyll(BChl)s are key pigments for the initial stage of photosynthetic processes, harvesting sunlight, transferring excited energy, and performing charge separation
The bciD gene-inactivated mutant of Cba. limnaeum was constructed and its pigments were analyzed. This mutant cells did not synthesize BChl e pigments, but accumulated BChl c species which were found in Cba. tepidum cells producing BChl c homologs, the composition of BChl c homologs was largely different between both cells
C20-unsubstituted BChl f pigments in the bchU mutant of Cba. limnaeum were almost the same as those of BChl e in the wild type cells [13], the present formation of S-rich epimers and isobutyl-rich homologs were ascribable to the alteration of the C7 substituent (7-methyl to formyl group) by the mutation

Summary

Introduction

Chlorophyll(Chl)s and bacteriochlorophyll(BChl)s are key pigments for the initial stage of photosynthetic processes, harvesting sunlight, transferring excited energy, and performing charge separation. All oxygenic photosynthetic organisms commonly possess Chl a (or 8-vinyl-Chl a in the case of Prochlorococcus). Among Chls b, d, and f, a gene encoding enzyme for formylation at C7 of Chl b has already been identified as chlorophyllide (Chlide) a oxygenase gene, CAO [5]. It was observed that the mutation of CAO gene in the green algae Chlamydomonas reinhardtii synthesized no Chl b, but exclusively Chl a, and a recombinant CAO enzyme converted the methyl group at C7 of Chlide a to the 7-formyl group of Chlide b via a hydroxymethyl group by two successive monooxygenations in vitro [6]. Elucidation of the genes encoding the oxidation to a formyl group is significant to understand the evolution of (B)Chl pigment biosynthesis as well as the photosynthetic mechanism

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