Biology subjects

Ledermann, R.

Publications and source records attributed to Ledermann, R..

3 recordsLinked to original sources

bioRxiv2026

DarHTG is a tripartite phage defence system for purine DNA base ADP-ribosylation

Bacteria evolved a variety of phage defence strategies to combat the constant onslaught of viral threats. DarTG toxin-antitoxin systems provide phage defence by ADP-ribosylation of bases in single-stranded DNA which interferes with viral replication and aborts phage infection. Here, we describe DarHTG, a system widespread among bacteria, that combines DarTG with a non-processive superfamily I helicase functioning as co-toxin for DarT and thereby enabling the ADP-ribosylation of double-stranded DNA. We identified two such DarHTG subclasses which catalyse the ADP-ribosylation of adenosine and guanosine bases, respectively, and reverse these modifications with structurally unique ARH superfamily antitoxins. Thus, DarHTG antitoxins expand the known macromolecular target spectrum of the ARH enzyme class to nucleic acids. These DarHTG systems confer resistance to a broad range of bacteriophages and can be triggered by the highly charged tails of viral single-stranded DNA-binding proteins, which occlude the active site of the ARH antitoxin.

microbiology↗

bioRxiv2021

Hopanoids confer robustness to physicochemical variability in the niche of the plant symbiont Bradyrhizobium diazoefficiens

Climate change poses a threat to soil health and agriculture, but the potential effects of climate change on soil bacteria that can help maintain soil health are understudied. Rhizobia are a group of bacteria that increase soil nitrogen content through a symbiosis with legume plants. The soil and symbiosis are potentially stressful environments, and the soil will likely become even more stressful as the climate changes. Many rhizobia within the bradyrhizobia clade, like Bradyrhizobium diazoefficiens, possess the genetic capacity to synthesize hopanoids, steroid-like lipids similar in structure and function to cholesterol. Hopanoids are known to protect against stresses relevant to the niche of B. diazoefficiens. Paradoxically, mutants unable to synthesize the extended class of hopanoids participate in similarly successful symbioses compared to the wild type, despite being delayed in root nodule initiation. Here, we show that in B. diazoefficiens, the in vitro growth defects of extended hopanoid deficient mutants can be at least partially compensated for by the physicochemical environment, specifically by optimal osmotic and divalent cation concentrations. Through biophysical measurements, we show that extended hopanoids confer robustness to environmental variability. These results help explain the discrepancy between previous in vitro and in planta results and indicate that hopanoids may provide a greater fitness advantage to rhizobia in the variable soil environment than the more controlled environment within root nodules. To improve the legume-rhizobia symbiosis through either bioengineering or strain selection, it will be important to consider the full lifecycle of rhizobia, from the soil to the symbiosis. ImportanceRhizobia, such as B. diazoefficiens, play an important role in the nitrogen cycle by making nitrogen gas bioavailable through symbiosis with legume plants. As climate change threatens soil health, this symbiosis has reentered the spotlight as a more sustainable source of soil nitrogen than the energy-intensive Haber-Bosch process. Efforts to use rhizobia as biofertilizers have been effective; however, long term integration of rhizobia into the soil community has been less successful. This work represents a small step towards improving the legume-rhizobia symbiosis by identifying a cellular component--hopanoid lipids--that confers robustness to environmental stresses rhizobia are likely to encounter in soil microenvironments as sporadic desiccation and flooding events become more common.

microbiology↗

bioRxiv2020

Lifestyle adaptations of Rhizobium from rhizosphere to symbiosis

By analyzing successive lifestyle stages of a model Rhizobium-legume symbiosis using mariner-based transposon insertion sequencing (INSeq), we have defined the genes required for rhizosphere growth, root colonization, bacterial infection, N2-fixing bacteroids and release from legume (pea) nodules. While only 27 genes are annotated as nif and fix in Rhizobium leguminosarum, we show 603 genetic regions (593 genes, 5 tRNAs and 5 RNA features) are required for the competitive ability to nodulate pea and fix N2. Of these, 146 are common to rhizosphere growth through to bacteroids. This large number of genes, defined as rhizosphere-progressive, highlights how critical successful competition in the rhizosphere is to subsequent infection and nodulation. As expected, there is also a large group (211) specific for nodule bacteria and bacteroid function. Nodule infection and bacteroid formation require genes for motility, cell envelope restructuring, nodulation signalling, N2 fixation, and metabolic adaptation. Metabolic adaptation includes urea, erythritol and aldehyde metabolism, glycogen synthesis, dicarboxylate metabolism and glutamine synthesis (GlnII). There are separate lifestyle adaptations specific to rhizosphere growth (17) and root colonization (23), distinct from infection and nodule formation. These results dramatically highlight the importance of competition at multiple stages of a Rhizobium-legume symbiosis. SignificanceRhizobia are soil-dwelling bacteria that form symbioses with legumes and provide biologically useable nitrogen as ammonium for the host plant. High-throughput DNA sequencing has led to a rapid expansion in publication of complete genomes for numerous rhizobia, but analysis of gene function increasingly lags gene discovery. Mariner-based transposon insertion sequencing (INSeq) has allowed us to characterize the fitness contribution of bacterial genes and determine those functionally important in a Rhizobium-legume symbiosis at multiple stages of development.

microbiology↗