Biosynthesis of the biomarker okenone

χ-ring formation

Research output: Contribution to journalArticle

8 Citations (Scopus)

Abstract

Purple sulfur bacteria (PSB) mainly occur in anoxic aquatic and benthic environments, where they play important roles in cycling carbon and sulfur. Many PSB characteristically produce the unique keto-carotenoid, okenone, which is important not only for its light absorption and photoprotection properties but also because of its diagenesis product, okenane, which is a biomarker for ancient sediments derived from anoxic environments. The specific methylation pattern of the χ-ring of okenane is unlikely to be formed by diagenetic processes and should therefore reflect an enzymatic activity from okenone biosynthesis. This study describes two enzymes that produce the χ-ring of okenone, the only structural element of okenone preserved in okenane. Genes encoding enzymes of carotenogenesis were identified in the draft genome sequence of an okenone-producing PSB, Thiodictyon sp. strain CAD16. Two divergently transcribed genes encoded a CrtY-type lycopene cyclase and a CrtU/CruE-type γ-carotene desaturase/methyltransferase. Expression of crtY in Escherichia coli showed that this gene encoded a lycopene cyclase that produced γ-carotene as the only product. Although the sequence of the γ-carotene desaturase/methyltransferase was more similar to CrtU sequences of green sulfur bacteria than to CruE sequences of cyanobacteria, expression of the crtU gene in Chlorobaculum tepidum showed that the enzyme produced carotenoids with χ-rings rather than φ-rings. Phylogenetic analysis of the carotene desaturase/methyltransferases revealed that enzymes capable of converting β-rings to χ-rings have independently evolved at least two times. These results indicate that it probably will not be possible to deduce the activity of carotene desaturase/methyltransferases solely from sequence data.

Original languageEnglish (US)
Pages (from-to)205-215
Number of pages11
JournalGeobiology
Volume10
Issue number3
DOIs
StatePublished - May 1 2012

Fingerprint

carotene 7,8-desaturase
methyltransferases
Chromatiaceae
biomarker
biomarkers
sulfur
biosynthesis
enzyme
lycopene
Thiodictyon
enzymes
bacterium
gene
carotenoid
carotenoids
Chlorobium tepidum
benthic zone
genes
photostability
photoprotection

All Science Journal Classification (ASJC) codes

  • Environmental Science(all)
  • Ecology, Evolution, Behavior and Systematics
  • Earth and Planetary Sciences(all)

Cite this

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title = "Biosynthesis of the biomarker okenone: χ-ring formation",
abstract = "Purple sulfur bacteria (PSB) mainly occur in anoxic aquatic and benthic environments, where they play important roles in cycling carbon and sulfur. Many PSB characteristically produce the unique keto-carotenoid, okenone, which is important not only for its light absorption and photoprotection properties but also because of its diagenesis product, okenane, which is a biomarker for ancient sediments derived from anoxic environments. The specific methylation pattern of the χ-ring of okenane is unlikely to be formed by diagenetic processes and should therefore reflect an enzymatic activity from okenone biosynthesis. This study describes two enzymes that produce the χ-ring of okenone, the only structural element of okenone preserved in okenane. Genes encoding enzymes of carotenogenesis were identified in the draft genome sequence of an okenone-producing PSB, Thiodictyon sp. strain CAD16. Two divergently transcribed genes encoded a CrtY-type lycopene cyclase and a CrtU/CruE-type γ-carotene desaturase/methyltransferase. Expression of crtY in Escherichia coli showed that this gene encoded a lycopene cyclase that produced γ-carotene as the only product. Although the sequence of the γ-carotene desaturase/methyltransferase was more similar to CrtU sequences of green sulfur bacteria than to CruE sequences of cyanobacteria, expression of the crtU gene in Chlorobaculum tepidum showed that the enzyme produced carotenoids with χ-rings rather than φ-rings. Phylogenetic analysis of the carotene desaturase/methyltransferases revealed that enzymes capable of converting β-rings to χ-rings have independently evolved at least two times. These results indicate that it probably will not be possible to deduce the activity of carotene desaturase/methyltransferases solely from sequence data.",
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Biosynthesis of the biomarker okenone : χ-ring formation. / Vogl, K.; Bryant, Donald Ashley.

In: Geobiology, Vol. 10, No. 3, 01.05.2012, p. 205-215.

Research output: Contribution to journalArticle

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