These two authors contributed equally.
Unique glycine-activated riboswitch linked to glycine–serine auxotrophy in SAR11
Article first published online: 3 OCT 2008
© 2008 The Authors. Journal compilation © 2008 Society for Applied Microbiology and Blackwell Publishing Ltd
Volume 11, Issue 1, pages 230–238, January 2009
How to Cite
Tripp, H. J., Schwalbach, M. S., Meyer, M. M., Kitner, J. B., Breaker, R. R. and Giovannoni, S. J. (2009), Unique glycine-activated riboswitch linked to glycine–serine auxotrophy in SAR11. Environmental Microbiology, 11: 230–238. doi: 10.1111/j.1462-2920.2008.01758.x
- Issue published online: 2 JAN 2009
- Article first published online: 3 OCT 2008
- Received 26 May, 2008; accepted 31 July, 2008.
The genome sequence of the marine bacterium ‘Candidatus Pelagibacter ubique’ and subsequent analyses have shown that while it has a genome as small as many obligate parasites, it nonetheless possesses a metabolic repertoire that allows it to grow as one of the most successful free-living cells in the ocean. An early report based on metabolic reconstruction indicated that SAR11 cells are prototrophs for all amino acids. However, here we report experimental evidence that ‘Cand. P. ubique’ is effectively auxotrophic for glycine and serine. With glucose and acetate added to seawater to supply organic carbon, the addition of 125 nM to 1.5 μM glycine to growth medium containing all other nutrients in excess resulted in a linear increase in maximum cell density from 1.14 × 106 cells ml−1 to 8.16 × 106 cells ml−1 (R2 = 0.992). Serine was capable of substituting for glycine at 1.5 μM. ‘Cand. P. ubique’ contains a glycine-activated riboswitch preceding malate synthase, an unusual genomic context that is conserved in the SAR11 group. Malate synthase plays a critical role in central metabolism by enabling TCA intermediates to be regenerated through the glyoxylate cycle. In vitro analysis of this riboswitch indicated that it responds solely to glycine but not close structural analogues, such as glycine betaine, malate, glyoxylate, glycolate, alanine, serine or threonine. We conclude that ‘Cand. P. ubique’ is therefore a glycine–serine auxotroph that appears to use intracellular glycine level to regulate its use of carbon for biosynthesis and energy. Comparative genomics and metagenomics indicate that these conclusions may hold throughout much of the SAR11 clade.