nodD2 of Rhizobium sp. NGR234 is involved in the repression of the nodABC operon

Abstract

SummaryTranscriptional regulators of the lysR family largely control the expression of bacterial symbiotic genes. Rhizobium sp. NGR234 contains at least four members of this family: two resemble nodD, while two others are more closely related to syrM. Part of the extremely broad host range of NGR234 can be attributed to nodD1, although the second gene shares a high degree of DNA sequence homology with nodD2 of R. fredii USDA191. A nodD2 mutant of NGR234 was constructed by insertional mutagenesis. This mutant (NGR⍀nodD2) was deficient in nitrogen fixation on Vigna unguiculata and induced pseudonodules on Tephrosia vogelii. Several other host plants were tested, but no correlation could be drawn between the phenotype and nodule morphology. Moreover, nodD2 has a negative effect on the production of Nod factors: mutation of this gene results in a fivefold increase in Nod factor production. Surprisingly, while the structure of Nod factors from free-living cultures of NGR⍀nodD2 remained unchanged, those from V. unguiculata nodules induced by the same strain are non-fucosylated and have a lower degree of oligomerization. In other words, developmental regulation of Nod factor production is also abolished in this mutant. Competitive RNA hybridizations, gene fusions and mobility shift assays confirmed that nodD2 downregulates expression of the nodABC operon.

Molecular Basis of Symbiotic Promiscuity

“…B. japonicum nolA and R. meliloti nolR mutants retain the ability to nodulate their respective hosts, albeit at lower efficiency, suggesting that fine-tuning of nod gene expression is required for optimal nodulation. Similarly, a nodD2 mutant of strain NGR234 fails to repress the expression of the nodABCIJnolOnoeI operon after initial flavonoid induction and, in contrast to the wildtype strain, is unable to form nitrogen-fixing nodules on V. unguiculata and Cajanus cajan (73).…”

Nodulation Gene Regulation inBradyrhizobium japonicum: a Unique Integration of Global Regulatory Circuits

“…Knight et al (37) demonstrated that the strong, constitutive expression of the nodulation genes results in defective and reduced nodulation phenotypes on host plants. It is not surprising, therefore, that in addition to positive activation the nod genes are also subject to negative regulation (10,15,21,39). In S. meliloti repression is mediated by NolR, while in Rhizobium sp.…”

“…strain NGR234 and B. elkani NodD2 acts as a repressor. Mutations to these repressors result in aberrant and delayed nodulation phenotypes (15,39). In B. japonicum, NolA and NodD2 form two key components in the negative regulation of the nodulation genes.…”

Flavonoid-Inducible Modifications to Rhamnan O Antigens Are Necessary for Rhizobium sp. Strain NGR234-Legume Symbioses

“…Various mutants of Rhizobium sp. strain NGR234 were constructed either by inserting an antibiotic resistance omega cassette into specific genes (giving NGR⍀target gene strains) or by deleting a restriction fragment internal to the target gene and replacing it with an omega antibiotic resistance cassette (resulting in NGR⌬target gene strains) using standard techniques (12). Promoter constructs cloned into the broad-host-range reporter vector pMP220 (52) were mobilized into NGR234 and its derivatives by triparental matings using pRK2013 as the helper plasmid (16).…”