Following (iso)flavonoid induction, nodulation genes of the symbiotic nitrogen-fixing bacterium Rhizobium sp. strain NGR234 elaborate a large family of lipooligosaccharidic Nod factors (NodNGR factors). When secreted into the rhizosphere of compatible legumes, these signal molecules initiate root hair deformation and nodule development. The nonreducing glucosamine residue of NodNGR factors are N acylated, N methylated, and mono- or biscarbamoylated, while position C-6 of the reducing extremity is fucosylated. This fucose residue is normally 2-O methylated and either sulfated or acetylated. Here we present an analysis of all acetylated NodNGR factors, which clearly shows that the acetate group may occupy position C-3 or C-4 of the fucose moiety. Disruption of the flavonoid-inducible nolL gene, which is preceded by a nod box, results in the synthesis of NodNGR factors that lack the 3- O - or 4- O -acetate groups. Interestingly, the nodulation capacity of the mutant NGRΩ nolL is not impaired, whereas introduction of the nod box:: nolL construct into the related strain Rhizobium fredii USDA257 extends the host range of this bacterium to Calopogonium caeruleum , Leucaena leucocephala , and Lotus halophilus . Nod factors produced by a USDA257(p nolL ) transconjugant were also acetylated. The nod box:: nolL construct was also introduced into ANU265 (NGR234 cured of its symbiotic plasmid), along with extra copies of the nodD1 gene. When permeabilized, these cells possessed acetyltransferase activity, although crude extracts did not.