Characterization of a partner switching system regulating c-di-GMP levels in Sinorhizobium meliloti: Implication in the synthesis of a novel exopolysaccharide

• Autores: Irene Baena Ropero
• Directores de la Tesis: Ildefonso Bonilla Mangas (dir. tes.), Javier LLoret Romero (dir. tes.)
• Lectura: En la Universidad Autónoma de Madrid ( España ) en 2016
• Idioma: inglés
• Tribunal Calificador de la Tesis: José Manuel Palacios Alberti (presid.), Rafael Rivilla Palma (secret.), Daniel Pérez Mendoza (voc.), José María Vinardell González (voc.), Jacob Malone (voc.)
• Materias:
  • Ciencias de la tierra y del espacio
    • Ciencias del suelo
      • Microbiología de suelos
• Enlaces
  • Tesis en acceso abierto en: Biblos-e Archivo
• Resumen

  • Sinorhizobium meliloti synthesizes a mixed-linkage (1→3) (1→4)-β-d-glucan (ML β-glucan) in response to high levels of cyclic diguanylate (c-di-GMP). Two proteins, BgsA and BgsB, are required for the synthesis, being BgsA the glucan synthase that senses c-di-GMP levels. The transcription of the bgsBA operon is dependent on the global regulator ExpR, which also forms part of the quorum sensing system Sin/ExpR. This study is focused on the characterization of an operon that regulates the synthesis of the ML-β glucan in S. meliloti. The operon was identified as a consequence of a screening of mutants affected in c-di-GMP metabolism.

    Among the six genes that constitute the operon, SMb20447 codes for a protein annotated as a diguanylate cyclase/phosphodiesterase. We demonstrated this protein is active as a diguanylate cyclase, and triggers the synthesis of the ML β-glucan.

    Unlike the bgsA gene, the transcription of the operon is not dependent on ExpR, thus forming and additional regulatory system. At least three of the gene products in the operon seem to form a partner switching system that resembles the one regulating Bacillus subtilis general stress response through sigma factors σB and σF. Using a genetic approach we determined that the proteins SMb20450 and SMb20451, which present putative phosphatase and kinase effector domains respectively, modulate the repressive activity of SMb20452, a STAS (sulphate transporter and anti-sigma factor antagonist) domain-containing protein.

    The system regulates the diguanylate cyclase activity of SMb20447 at the post-transcriptional level, probably through a direct interaction with SMb20452. The operon is well conserved in bacteria from the Rhizobiaceae family that present the bgsBA genes, indicating a similar role in these putative ML β-glucan producers.

    Regarding its biological role, the ML β-glucan proved to be very important for the attachment to alfalfa roots. However, its constitutive expression does not provide any advantage in relation to such attachment. On the other hand, this novel EPS did not present a symbiotic function, since it could not substitute either succynoglican (EPS I) nor galactoglucan (EPS II). Global transcriptomic analysis revealed that the SMb20447-SMb20452 operon might also regulate other processes like galactoglucan (EPS II) and siderophore production, adaptation to microaerobic environments, or cation transport.