Saccharomyces cerevisiae SCF Met30 ubiquitin-protein ligase controls cell cycle function and sulfur amino acid metabolism. We report here that the SCF Met30 complex mediates the transcriptional repression of the MET gene network by triggering degradation of the transcriptional activator Met4p when intracellular S-adenosylmethionine (AdoMet) increases. This AdoMet-induced Met4p degradation is dependent upon the 26S proteasome function. Unlike Met4p, the other components of the specific transcriptional activation complexes that are assembled upstream of the MET genes do not appear to be regulated at the protein level. We provide evidence that the interaction between Met4p and the F-box protein Met30p occurs irrespective of the level of intracellular AdoMet, suggesting that the timing of Met4p degradation is not controlled by its interaction with the SCF Met30 complex. We also demonstrate that Met30p is a short-lived protein, which localizes within the nucleus. Furthermore, transcription of the MET30 gene is regulated by intracellular AdoMet levels and is dependent upon the Met4p transcription activation function. Thus Met4p appears to control its own degradation by regulating the amount of assembled SCF Met30 ubiquitin ligase.

The ubiquitin system has been recently implicated in various aspects of transcriptional regulation, including proteasome-dependent degradation of transcriptional activators. In yeast, the activator Met4 is inhibited by the SCF(Met30) ubiquitin ligase, which recognizes and oligo-ubiquitylates Met4. Here, we demonstrate that in minimal media, Met4 is ubiquitylated and rapidly degraded in response to methionine excess, whereas in rich media, Met4 is oligo-ubiquitylated but remains stable. In the latter growth condition, oligo-ubiquitylated Met4 is not recruited to MET gene promoters, but is recruited to the SAM genes, which are required for production of S-adenosylmethionine, an unstable metabolite that is not present in rich medium. Thus, ubiquitylation not only regulates Met4 by distinct degradation-dependent and -independent mechanisms, but also controls differential recruitment of a single transcription factor to distinct promoters, thereby diversifying transcriptional activator specificity.

Activity of the Met4 transcription factor is antagonized by the SCFMet30 ubiquitin ligase by degradation-dependent and degradation-independent mechanisms, in minimal and rich nutrient conditions, respectively. In this study, we show that the heavy metal Cd2+ over-rides both mechanisms to enable rapid Met4-dependent induction of metabolic networks needed for production of the antioxidant and Cd2+-chelating agent glutathione. Cd2+ inhibits SCFMet30 activity through rapid dissociation of the F-box protein Met30 from the holocomplex. In minimal medium, dissociation of SCFMet30 complex is sufficient to impair the methionine-induced degradation of Met4. In rich medium, dissociation of the SCFMet30 complex is accompanied by a deubiquitylation mechanism that rapidly removes inhibitory ubiquitin moieties from Met4. Post-translational control of SCFMet30 assembly by a physiological stress to allow rapid induction of a protective gene expression program represents a novel mode of regulation in the ubiquitin system

The increasing interest for Galleria mellonella larvae as an infection model is evidenced by the number of papers reporting its use, which increases exponentially since the early 2010s. This popularity was initially linked to limitation of conventional animal models due to financial, technical and ethical aspects. In comparison, alternative models (e.g. models using Caenorhabditis elegans, Drosophila melanogaster or G. mellonella) were cheap, simple to use and not limited by ethical regulation. Since then, similar results have been established with G. mellonella model comparatively to vertebrates, and it is more and more often used as a robust model per se, not only as an alternative to the murine model. This review attempts to summarize the current knowledge supporting the development of this model, both on immunological and microbiological aspects. For that, we focus on investigation of virulence and new therapies for the most important pathogenic bacteria. We also discuss points out directions for standardization, as well as recent advances and new perspectives for monitoring host-pathogen interactions.