Monod et le phénomène de diauxie

International audienceDiauxie is at the origin of research that led Jacques Monod (1910-1976), François Jacob (1920-2013) and André Lwoff (1902-1994) to the Nobel Prize in Physiology or Medicine in 1965. It is a phenomenon of double bacterial growth observed by Monod in 1940 during his thesis, and which would have been discovered in 1900 by Frederic Diénert (1874-1948). In this article, a comparative analysis of Monod’s and Diénert’s work makes it clear that Monod is the discoverer of the diauxie and that Diénert did not observe this phenomenon.La diauxie est à l’origine de recherches qui ont mené Jacques Monod (1910-1976), François Jacob (1920-2013) et André Lwoff (1902-1994) au Prix Nobel de physiologie ou médecine en 1965. Il s’agit d’un phénomène de croissance bactérienne double observée par Monod en 1940 pendant sa thèse, et qui aurait été découvert en 1900 par Frederic Diénert (1874-1948). Dans cet article, l’analyse comparative des travaux de Monod et de Diénert permet de préciser que Monod est le découvreur de la diauxie et que Diénert n’a pas observé ce phénomène

Diauxic inhibition: Jacques Monod's Ignored Work

International audienceDiauxie is at the origin of research that led Jacques Monod (1910–1976), François Jacob (1920–2013), and André Lwoff (1902–1994) to win the Nobel Prize in Physiology or Medicine in 1965 for their description of the first genetic regulatory model. Diauxie is a term coined by Jacques Monod in 1941 in his doctoral dissertation that refers to microbial growth in two phases. In this article, we first examine Monod’s thesis to demonstrate how and why Monod interpreted diauxie as a phenomenon of enzyme inhibition or suppression of adaptive enzymes. We also briefly investigate prior enzyme suppression studies, before Monod’s work, which indicate that he is the first person to observe diauxic growth. Second, we analyse Monod’s post-thesis publications throughout his scientific career, revealing that diauxic inhibition was a significant part of Monod’s scientific activities and greatly fascinated Monod until the end of his life. Paradoxically, Monod’s work and interest on diauxic inhibition are still neglected in historical recounts, focused mostly on Monod’s enzymatic adaptation studies. Indeed, we uncovered a statement by Monod’s colleague, Lwoff, who transformed a quotation from Monod by replacing the word phenomenon with enzymatic adaptation, which we believe has influenced historians. Finally, we offer hypotheses to explain why Lwoff altered Monod’s statement

Etude du rôle des activateurs de transcription paralogues Aft1p et Aft2p dans la régulation du métabolisme du fer chez la levure Saccharomyces cerevisiae

Le fer est à la fois indispensable et toxique pour les cellules. Un contrôle strict de son métabolisme est nécessaire pour pourvoir aux besoins des cellules tout en évitant ses effets délétères. Chez la levure Saccharomyces cerevisiae, ce contrôle est assuré par les activateurs transcriptionnels paralogues Aft1p et Aft2p. En condition de carence en fer, Aft1p active la transcription des gènes du transport à haute affinité du fer extracellulaire en se fixant sur la séquence cis-régulatrice 5 -TGCACCC-3 . Le rôle d Aft2p est moins bien caractérisé ; il reconnaît in vitro la même séquence consensus qu Aft1p et active la transcription de plusieurs gènes régulés par Aft1p lorsqu il est surexprimé ou sous une forme mutée constitutivement active. Nous avons cherché à mieux comprendre le rôle d Aft2p en condition de carence en fer en identifiant ses gènes cibles et en caractérisant son mode d action, par une combinaison d expériences d analyses globales de transcriptome, de Northern blot et d immunoprécipitation de la chromatine réalisées avec les souches sauvage et délétées d AFT1 et/ou d AFT2. Nous avons montré qu Aft2p, mais pas Aft1p, active directement la transcription des gènes de l utilisation et du stockage du fer intracellulaire. Nous avons mis en évidence que la séquence cis-régulatrice des gènes régulés par Aft2p n est pas 5 -TGCACCC-3 , mais une séquence plus courte, 5 -(G/A)CACCC-3 . Aft2p joue également un rôle direct dans la régulation transcriptionnelle de plusieurs gènes du métabolisme du zinc, et il pourrait intervenir dans la régulation des gènes du métabolisme de l ergostérol et des acides gras. Par ailleurs, nous avons montré l existence d une régulation par le fer de la quantité des protéines Aft1p et Aft2p. La variation de quantité d Aft1p semble résulter de son autorégulation transcriptionnelle, alors que la variation de quantité d Aft2p implique une régulation post-transcriptionnelleIron is an essential nutrient, but its accumulation can be highly cytotoxic. Iron homeostasis must be tightly regulated in order to satisfy cell demand without damage. In the yeast Saccharomyces cerevisiae, this regulation is mediated by two paralogous transcription factors: Aft1p and Aft2p. Under iron starvation conditions, Aft1p activates the transcription of high affinity iron transport related genes by specific binding to the DNA consensus sequence 5 -TGCACCC-3 found in the promoters of these genes. The role of Aft2p remains poorly understood. Aft2p is capable of binding in vitro to the Aft1p DNA consensus sequence and it activates the transcription of some of the Aft1p target gene under specific conditions, such as overexpression, or expression of a gain-of-function allele. We aimed at further understanding the role of Aft2p in a physiological context, by identification of its target genes and characterisation of its mode of action. For this purpose, we performed global transcriptome analyses, Northern blot and chromatin immunoprecipitation experiments using wild type and aft1 and/or aft2 mutant strains grown under iron depleted conditions. We demonstrated that Aft2p directly activates the transcription of genes that are essential for the iron intracellular sub-compartimentalization and use, while Aft1p does not. We discovered that the consensus sequence in the promoter region of genes specifically activated by At2p was not the 5 -TGCACCC-3 sequence, but the shorter 5 -(G/A)CACCC- 3 . Aft2p plays also a direct role in the transcriptional regulation of some genes involved in zinc metabolism, and it could be involved in the regulation of ergosterol and fatty acid metabolism. We also showed that Aft1p and Aft2p proteins amount is iron-regulated. The modulation of Aft1p protein amount can be attributed to its own transcriptional regulation, while the modulation of Aft2p protein amount involves post-transcriptional regulationPARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Direct Activation of Genes Involved in Intracellular Iron Use by the Yeast Iron-Responsive Transcription Factor Aft2 without Its Paralog Aft1

The yeast Saccharomyces cerevisiae contains a pair of paralogous iron-responsive transcription activators, Aft1 and Aft2. Aft1 activates the cell surface iron uptake systems in iron depletion, while the role of Aft2 remains poorly understood. This study compares the functions of Aft1 and Aft2 in regulating the transcription of genes involved in iron homeostasis, with reference to the presence/absence of the paralog. Cluster analysis of DNA microarray data identified the classes of genes regulated by Aft1 or Aft2, or both. Aft2 activates the transcription of genes involved in intracellular iron use in the absence of Aft1. Northern blot analyses, combined with chromatin immunoprecipitation experiments on selected genes from each class, demonstrated that Aft2 directly activates the genes SMF3 and MRS4 involved in mitochondrial and vacuolar iron homeostasis, while Aft1 does not. Computer analysis found different cis-regulatory elements for Aft1 and Aft2, and transcription analysis using variants of the FET3 promoter indicated that Aft1 is more specific for the canonical iron-responsive element TGCACCC than is Aft2. Finally, the absence of either Aft1 or Aft2 showed an iron-dependent increase in the amount of the remaining paralog. This may provide additional control of cellular iron homeostasis

KlAft, the Kluyveromyces lactis Ortholog of Aft1 and Aft2, Mediates Activation of Iron-Responsive Transcription Through the PuCACCC Aft-Type Sequence

Iron homeostasis in fungi is regulated at the transcriptional level by two different mechanisms. It is mediated by a conserved GATA-type repressor in most fungi except in the yeast Saccharomyces cerevisiae, where it is controlled by the transcription activators Aft1 and Aft2. These activators are encoded by the paralogous genes AFT1 and AFT2, which result from the whole-genome duplication. Here, we explore regulation of iron homeostasis in the yeast Kluyveromyces lactis that diverged from S. cerevisiae before this event. We identify an ortholog of AFT1/AFT2, designated KlAFT, whose deletion leads to the inability to grow under iron limitation. We show with quantitative real-time PCR analysis that KlAft activates the transcription of all homologs of the Aft1-target genes involved in the iron transport at the cell surface in response to iron limitation. However, homologs of Aft2-specific target genes encoding intracellular iron transporters are regulated neither by KlAft nor by iron. Both bioinformatic and DNA binding and transcription analyses demonstrate that KlAft activates iron-responsive gene expression through the PuCACCC Aft-type sequence. Thus, K. lactis is the first documented species with a positive iron-transcriptional control mediated by only one copy of the Aft-type regulator. This indicates that this function was acquired before the whole-genome duplication and was then diversified into two regulators in S. cerevisiae