PHOSPHOLIPIDS OF RHIZOBIUM CONTAIN NODE-DETERMINED HIGHLY UNSATURATEDFATTY-ACID MOIETIES

In Rhizobium leguminosarum the nodABC and nodFEL operons are involved in the production of lipooligosaccharide signals, which mediate host s pecificity. A nodE-determined highly unsaturated C18.4 fatty acid (tra ns-2,trans-4,trans-6,cis-11-octadecatetraenoic acid) is essential for the ability of the signals to induce nodule primordia (Spaink, H. P., Sheeley, D. M., van Brussel, A. A. N., Glushka, J., York, W. S., Tak, T., Geiger, O., Kennedy, E. P., Reinhold, V. N., and Lugtenberg, B. J. J. (1991) Nature 354, 125-130) and preinfection thread structures (va n Brussel, A. A. N., Bakhuizen, R., van Spronsen, P. C., Spaink, H. P. , Tak, T., Lugtenberg, B. J. J., and Kijne, J. W. (1992) Science 257, 70-72) on the host plant Vicia sativa. We presently focus on the quest ion of how these lipooligosaccharide signals are synthesized in Rhizob ium. Here we show that after the induction of the nodFE genes, even in the absence of the nodABC genes, the trans-2,trans-4,trans-6,cis-11-o ctadecatetraenoic acid, which has a characteristic absorbance maximum of 303 nm, is synthesized; this shows that the biosynthesis of the unu sual fatty acid is not dependent on the synthesis of the lipooligosacc harides. This finding also suggests that the biosynthesis of the unusu al fatty acid is completed before it is linked to the sugar backbone o f the lipooligosaccharide. In an attempt to identify the lipid fractio n with which the unusual C18:4 fatty acid is associated, we found that it is linked to the sn-2 position of the phospholipids. Even when lip ooligosaccharide signals are produced in a wild type Rhizobium cell, a fraction of the unusual fatty acid is still bound to all major phosph olipids. These findings offer interesting possibilities. 1) The phosph olipids might be biosynthetic intermediates for the synthesis of lipoo ligosaccharide signals, and 2) phospholipids, containing nodFE-derived fatty acids, might have a signal function of their own.