Signal of Bradyrhizobium japonicum

The nodulation genes of rhizobia are regulated by the nodD gene product in response to host-produced flavonoids and appear to encode enzymes involved in the production of a lipo-chitose signal molecule required for infection and nodule formation. We have identified the nodZ gene of Bradyrhizobium japon&um, whose product is required for the addition of a 2-0-methylfucose residue to the terminal reducing N-acetylglu-cosamine of the nodulation signal. This substitution is essential for the biological activity of this molecule. Mutations in nodZ result in defective nodulation of siratro. Surprisingly, although nodZ clearly codes for nodulation function, it is not regulated by NodD and, indeed, shows elevated expression in planta. Therefore, nodZ represents a unique nodulation gene that is not under the control of NodD and yet is essential for the synthesis of an active nodulation signal. Leguminous plants are infected (nodulated) by the gram-negative, soil bacteria Rhizobium, Azorhizobium, and Bradyrhi-zobium species, which establish a nitrogen-fixing symbiosis within the roots. There is now considerable information avail-able concerning the genetic traits of these bacteria that are required for the establishment of nitrogen-fixing symbiosis.