Nod factors structurally are lipochitooligosaccharides (LCOs) that consist of an N-acetyl--glucosamine chain linked through β-1,4 linkage with a fatty acid of variable identity attached to a non reducing nitrogen in the backbone with various functional group substitutions at the terminal or non-terminal residues.
Nod factors are produced in complex mixtures differing in the following characteristics:
* Length of the chain can vary from three to six units of N-acetyl--glucosamine with the exception of M. loti which can produce Nod factors with two unit only.
* Presence or absence of strain-specific substitutions along the chain
* Identity of the fatty acid component
* Presence or absence of unsaturated fatty acids
Nod gene expression is induced by the presence of certain flavonoids in the soil, which are secreted by the plant and act as an attractant to bacteria and induce Nod factor production. Flavonoids activate NodD, a LysR family transcription factor, which binds to the nod box and initiates the transcription of the nod genes which encode the proteins necessary for the production of a wide range of LCOs.