The nodD gene of Rhizobium leguminosarum is autoregulatory and in the presence of plant exudate induces the nodA,B,C genes

Abstract

To analyse nod gene expression in Rhizobium leguminosarum, a broad host‐range lacZ protein fusion vector was constructed. Two protein fusions, nodC‐lacZ and nodD‐lacZ, were used to measure the regulation of expression of the promoters of the nodA,B,C and the nodD transcripts by measuring the induced levels of β‐galactosidase activity in R. leguminosarum. In the absence of plant root exudate the nodD‐lacZ hybrid was expressed but the nodC‐lacZ hybrid was not. The expression of the nodD‐lacZ hybrid was repressed in R. leguminosarum strains containing an intact cloned nodD gene indicating that the nodD gene is autoregulatory. The induction of the nodC‐lacZ hybrid required both the nodD gene and a component present in plant root exudate. Therefore the nodD gene acts both as a repressor and as an activator of gene expression. The nodD gene is adjacent to nodA and transcribed divergently from nodA,B,C with only ∼300 nucleotides between the coding regions of nodA and nodD. Within this intergenic region is a unique BclI site and, using nodC‐lacZ or nodD‐lacZ translational fusions with this BclI site as an end point, no induction of nodC‐lacZ or nodD‐lacZ was observed. Therefore the promoters of nodD and nodA,B,C overlap at least at this region, and the regulation of these overlapping promoters appears to be controlled by the nodD protein which becomes an activator only in the presence of a component from plant exudate.

Induction of Microbial Genes for Pathogenesis and Symbiosis by Chemicals from Root Border Cells

“…Under soil conditions in which free water can vary drastically millimeter by millimeter, the actual concentration of chemicals could be much higher or lower than that released by 30,000 border cells into 1 mL of water. Under the conditions used, however, the levels of inducing activity from border cells during a 16-h period of cocultivation were very similar to values obtained in other studies in which whole-root exudates were used (Rossen et al, 1985;Shearman et al, 1986;Coronado et al, 1995;McKhann et al, 1997).…”

“…It has been known for nearly a century that root exudates-the chemicals released from root systems into the soil-have profound effects on microorganisms in the rhizosphere (for review, see Curl and Truelove, 1986;Rovira, 1991). In recent years, specific sugars and phenolic compounds from plant exudates have been shown to induce microbial genes required for pathogenesis and symbiosis (Rossen et al, 1985;Stachel et al, 1985;Shearman et al, 1986;Peters and Long, 1988), and the cellular sources of such chemicals are now being examined (Maxwell and Phillips, 1990;Graham, 1991;Oommen et al, 1994;McKhann et al, 1997).…”

Three trans-acting regulatory functions control hydrogenase synthesis in Alcaligenes eutrophus

“…Finally, the hoxA-carrying HindIII fragment of pSUP202-2::pCH118 was used to replace the small HindIII fragment of pGE95, yielding plasmid pGE97. The hoxS'-'lacZ operon fusion plasmid pGE44 was constructed by cloning a 5.1-kb BamHI fragment of the hoxS operon (47) into the broad-host-range lacZ fusion vector pIJ1363 (35). (11).…”

Induction of Bradyrhizobium japonicum common nod genes by isoflavones isolated from Glycine max

“…nodD is a regulatory gene that is constitutively expressed and is required for induction of other nod genes (32)(33)(34). Recently, it has been reported that different nodD genes show different inducer specificities (34).…”