Organization of host-inducible transcripts on the symbiotic plasmid of <i>Rhizobium</i> sp. NGR234

In a systematic approach to identify genes involved in the early steps of the legume—Rhizobium symbiosis, we studied transcription patterns of symbiotic plasmid‐borne loci. A competitive hybridization procedure was used to identify DNA restriction fragments carrying genes whose expression is enhanced by plant root exudates or by purified flavonoids. Fragments containing induced genes were then located on the physical map of the 500 kb pNGR234a. New inducible loci as well as previously described genes were identified and their time course of induction determined. After initial induction, transcription of loci such as nodABC and the host‐specificity genes nodSU decreased to undetectable levels 24 h after incubation with purified flavonoids. In contrast, expression of other loci is detectable only after several hours of induction. Surprisingly, many genes remained transcribed in the nodD1 mutant suggesting the presence of other flavonoid‐dependent activators in NGR234. The hsnl region, which is involved in host specificity, was shown to carry several inducible but independently regulated transcripts. Sequencing analysis revealed several open reading frames whose products, based on sequence similarities, may be involved in L‐fucose metabolism and its adjunction to the Nod factors