Biomedical subjects

L Moulin

Publications and source records attributed to L Moulin.

4 recordsLinked to original sources

PubMed2001

Transcription attenuation associated with bacterial repetitive extragenic BIME elements.

Transcription attenuation comprises several processes that affect transcript elongation and transcription termination, and has an important role in regulating gene expression. In most cases, transcription attenuation is used as a regulatory mechanism that allows the cell to adjust protein synthesis levels in response to a specific signal. Here, by using a tRNA gene as a transcriptional reporter, we characterize a new type of transcription attenuation mechanism in Escherichia coli that involves bacterial interspersed mosaic elements (BIMEs), the main family of repetitive extragenic elements. The transcription termination factor Rho is required for attenuation in association with BIMEs, thus revealing a new role for Rho as a BIMEs-dependent global regulator of gene expression. By mutational analyses, we identified nucleotide determinants of BIMEs that are required for attenuation and showed that this process relies on a sequence-specific mechanism. Our data are consistent with a model in which BIMEs provoke a pause in RNA polymerase movement and Rho acts ultimately to terminate transcription. BIME-dependent transcription attenuation may be used as a means to differentially regulate expression of adjacent genes belonging to a single operon. BIMEs are dispersed in more than 250 operons such that attenuation can simultaneously affect expression of a large number of genes encoding unrelated proteins. This attenuation phenomenon, together with the known ability of BIMEs to stabilize upstream mRNA, reveals how dispersion of these abundant repetitive elements may affect gene regulation at the genome level.

Base Sequence↗

PubMed2001

Nodulation of legumes by members of the beta-subclass of Proteobacteria.

Members of the Leguminosae form the largest plant family on Earth, with around 18,000 species. The success of legumes can largely be attributed to their ability to form a nitrogen-fixing symbiosis with specific bacteria known as rhizobia, manifested by the development of nodules on the plant roots in which the bacteria fix atmospheric nitrogen, a major contributor to the global nitrogen cycle. Rhizobia described so far belong exclusively to the alpha-subclass of Proteobacteria, where they are distributed in four distinct phylogenetic branches. Although nitrogen-fixing bacteria exist in other proteobacterial subclasses, for example Herbaspirillum and Azoarcus from the phylogenetically distant beta-subclass, none has been found to harbour the nod genes essential for establishing rhizobial symbiosis. Here we report the identification of proteobacteria from the beta-subclass that nodulate legumes. This finding shows that the ability to establish a symbiosis with legumes is more widespread in bacteria than anticipated to date.

Betaproteobacteria↗

PubMed2001

Nod genes and Nod signals and the evolution of the Rhizobium legume symbiosis.

The establishment of the nitrogen-fixing symbiosis between rhizobia and legumes requires an exchange of signals between the two partners. In response to flavonoids excreted by the host plant, rhizobia synthesize Nod factors (NFs) which elicit, at very low concentrations and in a specific manner, various symbiotic responses on the roots of the legume hosts. NFs from several rhizobial species have been characterized. They all are lipo-chitooligosaccharides, consisting of a backbone of generally four or five glucosamine residues N-acylated at the non-reducing end, and carrying various O-substituents. The N-acyl chain and the other substituents are important determinants of the rhizobial host specificity. A number of nodulation genes which specify the synthesis of NFs have been identified. All rhizobia, in spite of their diversity, possess conserved nodABC genes responsible for the synthesis of the N-acylated oligosaccharide core of NFs, which suggests that these genes are of a monophyletic origin. Other genes, the host specific nod genes, specify the substitutions of NFs. The central role of NFs and nod genes in the Rhizobium-legume symbiosis suggests that these factors could be used as molecular markers to study the evolution of this symbiosis. We have studied a number of NFs which are N-acylated by alpha,beta-unsaturated fatty acids. We found that the ability to synthesize such NFs does not correlate with taxonomic position of the rhizobia. However, all rhizobia that produce NFs such nodulate plants belonging to related tribes of legumes, the Trifolieae, Vicieae, and Galegeae, all of them being members of the so-called galegoid group. This suggests that the ability to recognize the NFs with alpha-beta-unsaturated fatty acids is limited to this group of legumes, and thus might have appeared only once in the course of legume evolution, in the galegoid phylum.

Biological Evolution↗

PubMed1996

[Not Available].

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Europe↗