Abstract
Soil-dwelling, nitrogen-fixing rhizobia signal their presence to legume hosts by secreting lipo-chitooligomers (LCOs) that are decorated with a variety of chemical substituents. It has long been assumed, but never empirically shown, that the LCO backbone is synthesized first by NodC, NodB, and NodA, followed by addition of one or more substituents by other Nod proteins. By analyzing a collection of in-frame deletion mutants of key nod genes in the bacterium Rhizobium sp. IRBG74 by mass spectrometry, we were able to shed light on the possible substitution order of LCO decorations, and we discovered that the prevailing view is probably erroneous. We found that most substituents could be transferred to a short chitin backbone prior to acylation by NodA, which is probably one of the last steps in LCO biosynthesis. The existence of substituted, short chitin oligomers offers new insights into symbiotic plant–microbe signaling.
Highlights
Rhizobia are soil-dwelling, Gram-negative bacteria that can establish a nitrogen-fixing symbiosis with legumes [1]
The LCOs secreted into the culture medium by overexpressing strains are not expected to differ significantly from those found in the membranes of wild-type rhizobia [14]
Arabinosylation never occurred in the absence of fucosylation, and more than half of the LCO-IV molecules were carbamoylated without any other decoration
Summary
Rhizobia are soil-dwelling, Gram-negative bacteria that can establish a nitrogen-fixing symbiosis with legumes [1]. In this association, legumes develop specialized organs, root nodules, to host the bacteria under low oxygen tensions that are favorable to nitrogen fixation and provide hydrocarbons (malate) to the microorganism to sustain, in particular, the nitrogenase activity. The enzymes responsible for Nod factor biosynthesis are encoded by nod, noe, and nol genes, which are located either on symbiotic plasmids or in genomic islands. These genes are found under the control of regulatory elements, called nod boxes [5], which are recognized by the transcription factor NodD [6].
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