Identification of a methylase required for 2-methylhopanoid production and implications for the interpretation of sedimentary hopanes

Paula V. Welander, Maureen L. Coleman, Alex L. Sessions, Roger E. Summons, Dianne K. Newman

    Research output: Contribution to journalArticle

    145 Citations (Scopus)

    Abstract

    The rise of atmospheric oxygen has driven environmental change and biological evolution throughout much of Earth's history and was enabled by the evolution of oxygenic photosynthesis in the cyanobacteria. Dating this metabolic innovation using inorganic proxies from sedimentary rocks has been difficult and one important approach has been to study the distributions of fossil lipids, such as steranes and 2-methylhopanes, as biomarkers for this process. 2-methylhopanes arise from degradation of 2-methylbacteriohopanepolyols (2-MeBHPs), lipids thought to be synthesized primarily by cyanobacteria. The discovery that 2-MeBHPs are produced by an anoxygenic phototroph, however, challenged both their taxonomic link with cyanobacteria and their functional link with oxygenic photosynthesis. Here, we identify a radical SAM methylase encoded by the hpnP gene that is required for methylation at the C-2 position in hopanoids. This gene is found in several, but not all, cyanobacteria and also in α -proteobacteria and acidobacteria. Thus, one cannot extrapolate from the presence of 2-methylhopanes alone, in modern environments or ancient sedimentary rocks, to a particular taxonomic group or metabolism. To understand the origin of this gene, we reconstructed the evolutionary history of HpnP. HpnP proteins from cyanobacteria, Methylobacterium species, and other α-proteobacteria form distinct phylogenetic clusters, but the branching order of these clades could not be confidently resolved. Hence,it is unclear whether HpnP, and 2-methylhopanoids, originated first in the cyanobacteria. In summary, existing evidence does not support the use of 2-methylhopanes as biomarkers for oxygenic photosynthesis.

    Original languageEnglish
    Pages (from-to)8537-8542
    Number of pages6
    JournalProceedings of the National Academy of Sciences of the United States of America
    Volume107
    Issue number19
    DOIs
    Publication statusPublished - 11 May 2010

    Keywords

    • Biomarkers
    • Cyanobacteria
    • Hopanoids
    • Phylogeny
    • Radical SAM

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