Biosynthesis of 2-methylbacteriohopanepolyols by an anoxygenic phototroph

doi:10.1073/pnas.0704912104

Biosynthesis of 2-methylbacteriohopanepolyols by an anoxygenic phototroph

Divisions of *Geological and Planetary Sciences and
^§Biology, California Institute of Technology and
^¶Howard Hughes Medical Institute, Pasadena, CA 91125; and
^‡Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139

Edited by John M. Hayes, Woods Hole Oceanographic Institution, Woods Hole, MA, and approved August 3, 2007 (received for review May 25, 2007)

Abstract

Sedimentary 2-methyhopanes have been used as biomarker proxies for cyanobacteria, the only known bacterial clade capable of oxygenic photosynthesis and the only group of organisms found thus far to produce abundant 2-methylbacteriohopanepolyols (2-MeBHPs). Here, we report the identification of significant quantities of 2-MeBHP in two strains of the anoxygenic phototroph Rhodopseudomonas palustris. Biosynthesis of 2-MeBHP can occur in the absence of O₂, deriving the C-2 methyl group from methionine. The relative abundance of 2-MeBHP varies considerably with culture conditions, ranging from 13.3% of total bacteriohopanepolyol (BHP) to trace levels of methylation. Analysis of intact BHPs reveals the presence of methylated bacteriohopane-32,33,34,35-tetrol but no detectable methylation of 35-aminobacteriohopane-32,33,34-triol. Our results demonstrate that an anoxygenic photoautotroph is capable of generating 2-MeBHPs and show that the potential origins of sedimentary 2-methylhopanoids are more diverse than previously thought.

Footnotes

^‖To whom correspondence should be addressed at:
Massachusetts Institute of Technology, Building 68-380, 77 Massachusetts Avenue, Cambridge, MA 02139.
E-mail: dkn{at}mit.edu
Author contributions: S.E.R., A.L.S., and D.K.N. designed research; S.E.R. and R.E.S. performed research; S.E.R. and R.E.S. contributed new reagents/analytic tools; S.E.R., A.L.S., R.E.S., and D.K.N. analyzed data; and S.E.R., A.L.S., and D.K.N. wrote the paper.
↵ ^†Deceased August 25, 2007.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/cgi/content/full/0704912104/DC1.
Abbreviations:

2-MeBHP,

2-methylbacteriohopanepolyol;

BHP,

bacteriohopanepolyol;

GC-MS,

gas chromatography-mass spectrometry.