Adaptation of green sulfur bacteria to limited-light conditions by transcriptional regulation of two C3-vinyl hydratase genes, bchF and bchV

Jiro Haradaa, Misato Teramurab, Tadashi Mizoguchib, Yusuke Tsukatanic, Ken Yamamotoa, Hitoshi Tamiakib
aDepartment of Medical Biochemistry, Kurume University School of Medicine, Fukuoka 830-0011, Japan; bGraduate School of Life Sciences, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan; cEarth-Life Science Institute, Tokyo Institute of Technology, Meguro, Tokyo 152-8550, Japan

Green sulfur bacteria are strictly anaerobic and photoautotroph, and are often found in sulfur-rich environments of very low light, such as the bottom of stratified lakes. To adapt to such environments, these bacteria have evolved efficient light-harvesting antenna complexes called as chlorosomes, which comprise self-aggregates of bacteriochlorophyll (BChl) c, d, or e. These pigments possess a hydroxy group at the C31 position that produces a chiral center with R- or S-stereochemistry and serves as a connecting moiety for their self-aggregation. Compositions of R- and S-epimeric BChl c, d, and e are sensitive to growth conditions of bacterial cells. Under limited-light conditions, an increase in the ratio of S-epimeric pigments occurs, and chlorosomes exhibit a red shift of their absorption band toward that of BChl a in the CsmA-baseplate, leading to advantages for Förster-type excitation energy transfer. The green sulfur bacterium Chlorobaculum tepidum synthesizes BChl c as the chlorosomal pigment, and also a protein-binding BChl a. This bacterium carries the two homologous genes bchF and bchV for the C3-vinyl hydratase BchF which is involved in the biosynthesis of purple bacterial BChl a. In the present study, we constructed deletion mutants of each of these genes and determined their functions [1].

In pigment analyses, the bchF-mutant showed a higher ratio of S-epimeric BChl c than the wild-type strain. The heightened proportion of S-stereoisomers in this mutant was remarkable at lower light intensities and induced a red shift of the chlorosomal Qy absorption band, reflecting adaptation to low light. On the other hands, the analysis for the bchV mutant showed that BchF catalyzed the hydration of C3-vinyl groups in BChl c biosynthesis, predominantly to R-epimers, but had a less activity for hyper-alkylated substrates at C81 position. It was also revealed that the BchF enzyme had a fignificant role in the BChl a biosynthesis. The semi-quantitative reverse transcriptase PCR analyses showed that both the bchF and bchV genes were transcriptionally sensitive to light intensity, and their upregulations at the low-light were significant for BChls c and a biosynthetic pathways. We will discuss about the roles of BchF and BchV for the alteration of environmental light conditions.

Reference

[1] J. Harada, M. Teramura, T. Mizoguchi, Y. Tsukatani, K. Yamamoto, and H. Tamiaki, Mol. Microbiol. 2015, 98, 1184–1198.