Host-specific regulation of symbiotic nitrogen fixation in Rhizobium leguminosarum biovar trifolii.

Strains of Rhizobium leguminosarum bv. trifolii (Rlt) able to form effective nodules on Trifolium ambiguum (Caucasian clover, CC) form ineffective nodules on Trifolium repens (white clover, WC), whereas strains that form effective nodules on WC usually do not nodulate CC. Here, we investigate the genetic basis of the host-specific nitrogen-fixation phenotype of CC rhizobia. A cosmid library of the symbiotic plasmid from the WC rhizobium strain Rlt NZP514 was introduced into the CC rhizobium strain Rlt ICC105. An 18 kb Asp718 fragment containing the nifABHDKEN and fixABCX genes of NZP514 that imparted the Fix(+) phenotype was identified. Tn5 mutagenesis of this region revealed that the nifHDKEN, fixABC and nifB genes were required for the Fix(+) phenotype, but that the nifA gene was not. Introduction of several plasmids containing NZP514 nif/fix genes into an ICC105 nifA mutant strain demonstrated that the NifA protein of ICC105 was able to activate expression of the NZP514 nif/fix genes but not the ICC105 nif/fix genes in WC nodules. Reporter gene fusion studies showed that the host-specific regulation of the nif/fix genes depended on the DNA region between the promoters of the divergently transcribed nifH and fixA genes. We hypothesize that a protein acting either in response to a host-specific signal or in the absence of such a signal is able to bind upstream of the NifA-binding sites and interact with NifA to prevent it activating nif/fix gene expression.

Miller SH ，Elliot RM ，Sullivan JT ，Ronson CW ... -  《MICROBIOLOGY-SGM》

Rhizobium leguminosarum bv. trifolii NodD2 Enhances Competitive Nodule Colonization in the Clover-Rhizobium Symbiosis.

Establishment of the symbiotic relationship that develops between rhizobia and their legume hosts is contingent upon an interkingdom signal exchange. In response to host legume flavonoids, NodD proteins from compatible rhizobia activate expression of nodulation genes that produce lipochitin oligosaccharide signaling molecules known as Nod factors. Root nodule formation commences upon legume recognition of compatible Nod factor. was previously considered to contain one copy of ; here, we show that some strains of the (clover) microsymbiont bv. trifolii contain a second copy designated genes were present in 8 out of 13 strains of bv. trifolii, but were absent from the genomes of 16 bv. viciae strains. Analysis of single and double and mutants in bv. trifolii strain TA1 revealed that NodD2 was functional and enhanced nodule colonization competitiveness. However, NodD1 showed significantly greater capacity to induce gene expression and infection thread formation. Clover species are either annual or perennial and this phenological distinction is rarely crossed by individual bv. trifolii microsymbionts for effective symbiosis. Of 13 strains with genome sequences available, 7 of the 8 effective microsymbionts of perennial hosts contained , whereas the 3 microsymbionts of annual hosts did not. We hypothesize that NodD2 inducer recognition differs from NodD1, and NodD2 functions to enhance competition and effective symbiosis, which may discriminate in favor of perennial hosts. Establishment of the rhizobium-legume symbiosis requires a highly specific and complex signal exchange between both participants. Rhizobia perceive legume flavonoid compounds through LysR-type NodD regulators. Often, rhizobia encode multiple copies of , which is one determinant of host specificity. In some species of rhizobia, the presence of multiple copies of NodD extends their symbiotic host-range. Here, we identified and characterized a second copy of present in some strains of the clover microsymbiont bv. trifolii. The second gene contributed to the competitive ability of the strain on white clover, an important forage legume. A screen for strains containing could be utilized as one criterion to select strains with enhanced competitive ability for use as inoculants for pasture production.

Ferguson S ，Major AS ，Sullivan JT ，Bourke SD ，Kelly SJ ，Perry BJ ，Ronson CW ... -  《-》

Host-dependent symbiotic efficiency of Rhizobium leguminosarum bv. trifolii strains isolated from nodules of Trifolium rubens.

Marek-Kozaczuk M ，Wdowiak-Wróbel S ，Kalita M ，Chernetskyy M ，Deryło K ，Tchórzewski M ，Skorupska A ... -  《-》

Molecular linkage of the nif/fix and nod gene regions in Rhizobium leguminosarum biovar trifolii.

Nucleotide sequence analysis of a 2.5kb region downstream of the nifA gene from Rhizobium leguminosarum biovar trifolii has resulted in linkage, at the DNA sequence level, of the nifEN, nifHDK, fixABCX, nifA gene cluster with the nodEF, nodD, nodABCIJ genes. Four genes have been identified within this intervening region. Immediately 3' to the nifA gene is the nifB gene and the nifB-linked ferredoxin-encoding fdxN gene. Downstream of fdxN in R. leguminosarum bv. trifolii and in Rhizobium meliloti, we have identified an open reading frame which has not been described previously and which we propose to designate fixU. Downstream of fixU in R. leguminosarum bv. trifolii is a nod gene, nodT, which is contiguous with nodJ (B. Surin et al., manuscript in preparation). As a result of this study, the linkage relationships of 22 symbiotic genes spanning a 24 kb region of the symbiotic plasmid from R. leguminosarum bv. trifolii are now known.

Iismaa SE ，Ealing PM ，Scott KF ，Watson JM ... -  《MOLECULAR MICROBIOLOGY》

Expression of the symbiotic plasmid from Rhizobium leguminosarum biovar trifolii in Sphingobacterium multivorum.

Fenton M ，Jarvis BD 《CANADIAN JOURNAL OF MICROBIOLOGY》