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 hpnP gene coding for the radical SAM methylase protein that acts on the C2 position of hopanoids, was deleted from the filamentous cyanobacterium Nostoc punctiforme ATCC 29133S. The resulting ΔhpnP mutant lacked all 2-methylhopanoids, but was found to produce much higher levels of two bacteriohopanepentol isomers compared to the wild type. Growth rates of ΔhpnP 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 ΔhpnP mutant were similar to those of vegetative cell-dominated cultures. However, the ΔhpnP 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 ΔhpnP mutant under stress conditions, compared to the wild type, suggested that the absence of 2-methylhopanoids increases cellular metabolic rates under stress conditions.