Lack of Methylated Hopanoids Renders the Cyanobacterium Nostoc punctiforme Sensitive to Osmotic and pH Stress.

To investigate the function of 2-methylhopanoids in modern cyanobacteria, the gene coding for the radical -adenosyl methionine (SAM) methylase protein that acts on the C-2 position of hopanoids was deleted from the filamentous cyanobacterium ATCC 29133S. The resulting Δ mutant lacked all 2-methylhopanoids but was found to produce much higher levels of two bacteriohopanepentol isomers than the wild type. Growth rates of the Δ mutant cultures were not significantly different from those of the wild type under standard growth conditions. Akinete formation was also not impeded by the absence of 2-methylhopanoids. The relative abundances of the different hopanoid structures in akinete-dominated cultures of the wild-type and Δ mutant strains were similar to those of vegetative cell-dominated cultures. However, the Δ mutant was found to have decreased growth rates under both pH and osmotic stress, confirming a role for 2-methylhopanoids in stress tolerance. Evidence of elevated photosystem II yield and NAD(P)H-dependent oxidoreductase activity in the Δ mutant under stress conditions, compared to the wild type, suggested that the absence of 2-methylhopanoids increases cellular metabolic rates under stress conditions. As the first group of organisms to develop oxygenic photosynthesis, are central to the evolutionary history of life on Earth and the subsequent oxygenation of the atmosphere. To investigate the origin of cyanobacteria and the emergence of oxygenic photosynthesis, geobiologists use biomarkers, the remnants of lipids produced by different organisms that are found in geologic sediments. 2-Methylhopanes have been considered indicative of cyanobacteria in some environmental settings, with the parent lipids 2-methylhopanoids being present in many contemporary cyanobacteria. We have created a Δ mutant strain that does not produce 2-methylhopanoids to assess the influence of 2-methylhopanoids on stress tolerance. Increased metabolic activity in the mutant under stress indicates compensatory alterations in metabolism in the absence of 2-methylhopanoids.