High hopanoid/total lipids ratio in Frankia mycelia is not related to the nitrogen status

Vesicles are specific Frankia structures which are produced under nitrogen- limiting culture conditions. Hopanoids are the most abundant lipids in thes e vesicles and are believed to protect the nitrogenase against oxygen. The amounts and quality of each hopanoid were estimated in different Frankia st rains cultivated under nitrogen-depleted and nitrogen-replete conditions in order to detect a possible variation. Studied Frankia strains nodulating E leagnus were phylogenetically characterized by analysis of the nifD-K inter genic region as closely related to genomic species 4 and 5. Phylogeneticall y different strains belonging to three infectivity groups were cultivated i n the same medium with and without nitrogen source for 10 d before hopanoid content analysis by HPLC. Four hopanoids together accounted for 23-87 % an d 15-87 %. of the total lipids under nitrogen-replete and nitrogen-depleted culture conditions, respectively. Two of the hopanoids found, bacteriohopa netetrols and their phenylacetic acid esters, have previously been describe d in Frankia. Two new hopanoids, mortean-29-ol and a bacteriohopanetetrol p ropionate, have also been identified. The moretan-29-ol and bacteriohopanet etrols were found to be the most abundant hopanoids whereas the bacteriohop anetetrol propionate and phenylacetates were present at a concentration clo se to the limit of detection. The ratio of (bacteriohopanetetrols + moretan -29-ol)/(total lipids) varied in most of the strains between nitrogen-deple ted and nitrogen-replete culture conditions. In most of the strains, the ho panoid content wits found to be slightly higher under nitrogen-replete cond itions than under nitrogen-depleted conditions. These results suggest that remobilization, rather than neosynthesis of hopanoids, is implicated in ves icle formation in Frankia under nitrogen-depleted conditions.