Novel Mono-, Di-, and Trimethylornithine Membrane Lipids in Northern Wetland Planctomycetes

Abstract

dNorthern peatlands represent a significant global carbon store and commonly originate from Sphagnum moss-dominated wetlands. These ombrotrophic ecosystems are rain fed, resulting in nutrient-poor, acidic conditions. Members of the bacterial phylum Planctomycetes are highly abundant and appear to play an important role in the decomposition of Sphagnum-derived litter in these ecosystems. High-performance liquid chromatography coupled to high-resolution accurate-mass mass spectrometry (HPLC-HRAM/MS) analysis of lipid extracts of four isolated planctomycetes from wetlands of European north Russia revealed novel ornithine membrane lipids (OLs) that are mono-, di-, and trimethylated at the -nitrogen position of the ornithine head group. Nuclear magnetic resonance (NMR) analysis of the isolated trimethylornithine lipid confirmed the structural identification. Similar fatty acid distributions between mono-, di-, and trimethylornithine lipids suggest that the three lipid classes are biosynthetically linked, as in the sequential methylation of the terminal nitrogen in phosphatidylethanolamine to produce phosphatidylcholine. The mono-, di-, and trimethylornithine lipids described here represent the first report of methylation of the ornithine head groups in biological membranes. Various bacteria are known to produce OLs under phosphorus limitation or fatty-acid-hydroxylated OLs under thermal or acid stress. The sequential methylation of OLs, leading to a charged choline-like moiety in the trimethylornithine lipid head group, may be an adaptation to provide membrane stability under acidic conditions without the use of scarce phosphate in nutrient-poor ombrotrophic wetlands.

OlsG (Sinac_1600) Is an Ornithine Lipid N-Methyltransferase from the Planctomycete Singulisphaera acidiphila

“…4H). TMOLs extracted from both species showed very similar fragmentation patterns presenting the characteristic TMOL head group fragment m/z 116.07 (11) and only differed in their fatty acid composition (Fig. 4, F and I).…”

“…In recent years several OL modifications have been identified, all of them hydroxylations (3, 6 -9). Therefore it came as a surprise when a novel OL structure, N-methylated at the ␦-amino group of the ornithine headgroup, was described in several species belonging to the planctomycetes (11).…”

“…Recently, the presence of N-methylated OL derivatives has been described in a few planctomycetes (11) (Fig. 1).…”

“…Column temperature was held constant at 40°C (17). Identification of OL and TMOL species occurred by comparison with previously published fragmentation patterns (11,18).…”

“…Recently, the presence of N-methylated OLs has been reported in membranes of Singulisphaera acidiphila, Singulisphaera rosea, Telmatocola sphagniphila, and Gemmata-like strain SP5, all of them planctomycetes isolated from a bog in Northern Siberia (11). Here we present the identification and characterization of the OL N-methyltransferase OlsG from S. acidiphila, responsible for the biosynthesis of a series of novel OLs, mono-, di-, and trimethylated on the ␦-nitrogen of the ornithine head group.…”