Diauxic Inhibition: Jacques Monod's Ignored Work.

Diauxie 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.

Ostreococcus tauri is a new model green alga for studying iron metabolism in eukaryotic phytoplankton.

BACKGROUND: Low iron bioavailability is a common feature of ocean surface water and therefore micro-algae developed original strategies to optimize iron uptake and metabolism. The marine picoeukaryotic green alga Ostreococcus tauri is a very good model for studying physiological and genetic aspects of the adaptation of the green algal lineage to the marine environment: it has a very compact genome, is easy to culture in laboratory conditions, and can be genetically manipulated by efficient homologous recombination. In this study, we aimed at characterizing the mechanisms of iron assimilation in O. tauri by combining genetics and physiological tools. Specifically, we wanted to identify and functionally characterize groups of genes displaying tightly orchestrated temporal expression patterns following the exposure of cells to iron deprivation and day/night cycles, and to highlight unique features of iron metabolism in O. tauri, as compared to the freshwater model alga Chalamydomonas reinhardtii. RESULTS: We used RNA sequencing to investigated the transcriptional responses to iron limitation in O. tauri and found that most of the genes involved in iron uptake and metabolism in O. tauri are regulated by day/night cycles, regardless of iron status. O. tauri lacks the classical components of a reductive iron uptake system, and has no obvious iron regulon. Iron uptake appears to be copper-independent, but is regulated by zinc. Conversely, iron deprivation resulted in the transcriptional activation of numerous genes encoding zinc-containing regulation factors. Iron uptake is likely mediated by a ZIP-family protein (Ot-Irt1) and by a new Fea1-related protein (Ot-Fea1) containing duplicated Fea1 domains. The adaptation of cells to iron limitation involved an iron-sparing response tightly coordinated with diurnal cycles to optimize cell functions and synchronize these functions with the day/night redistribution of iron orchestrated by ferritin, and a stress response based on the induction of thioredoxin-like proteins, of peroxiredoxin and of tesmin-like methallothionein rather than ascorbate. We briefly surveyed the metabolic remodeling resulting from iron deprivation. CONCLUSIONS: The mechanisms of iron uptake and utilization by O. tauri differ fundamentally from those described in C. reinhardtii. We propose this species as a new model for investigation of iron metabolism in marine microalgae.

Different iron sources to study the physiology and biochemistry of iron metabolism in marine micro-algae.

We compared ferric EDTA, ferric citrate and ferrous ascorbate as iron sources to study iron metabolism in Ostreococcus tauri, Phaeodactlylum tricornutum and Emiliania huxleyi. Ferric EDTA was a better iron source than ferric citrate for growth and chlorophyll levels. Direct and indirect experiments showed that iron was much more available to the cells when provided as ferric citrate as compared to ferric EDTA. As a consequence, growth media with iron concentration in the range 1-100 nM were rapidly iron-depleted when ferric citrate-but not ferric EDTA was the iron source. When cultured together, P. tricornutum cells overgrew the two other species in iron-sufficient conditions, but E. huxleyi was able to compete other species in iron-deficient conditions, and when iron was provided as ferric citrate instead of ferric EDTA, which points out the critical influence of the chemical form of iron on the blooms of some phytoplankton species. The use of ferric citrate and ferrous ascorbate allowed us to unravel a kind of regulation of iron uptake that was dependent on the day/night cycles and to evidence independent uptake systems for ferrous and ferric iron, which can be regulated independently and be copper-dependent or independent. The same iron sources also allowed one to identify molecular components involved in iron uptake and storage in marine micro-algae. Characterizing the mechanisms of iron metabolism in the phytoplankton constitutes a big challenge; we show here that the use of iron sources more readily available to the cells than ferric EDTA is critical for this task.

A comparative study of iron uptake mechanisms in marine microalgae: iron binding at the cell surface is a critical step.

We investigated iron uptake mechanisms in five marine microalgae from different ecologically important phyla: the diatoms Phaeodactylum tricornutum and Thalassiosira pseudonana, the prasinophyceae Ostreococcus tauri and Micromonas pusilla, and the coccolithophore Emiliania huxleyi. Among these species, only the two diatoms were clearly able to reduce iron, via an inducible (P. tricornutum) or constitutive (T. pseudonana) ferrireductase system displaying characteristics similar to the yeast (Saccharomyces cerevisiae) flavohemoproteins proteins. Iron uptake mechanisms probably involve very different components according to the species, but the species we studied shared common features. Regardless of the presence and/or induction of a ferrireductase system, all the species were able to take up both ferric and ferrous iron, and iron reduction was not a prerequisite for uptake. Iron uptake decreased with increasing the affinity constants of iron-ligand complexes and with increasing ligand-iron ratios. Therefore, at least one step of the iron uptake mechanism involves a thermodynamically controlled process. Another step escapes to simple thermodynamic rules and involves specific and strong binding of ferric as well as ferrous iron at the cell surface before uptake of iron. Binding was paradoxically increased in iron-rich conditions, whereas uptake per se was induced in all species only after prolonged iron deprivation. We sought cell proteins loaded with iron following iron uptake. One such protein in O. tauri may be ferritin, and in P. tricornutum, Isip1 may be involved. We conclude that the species we studied have uptake systems for both ferric and ferrous iron, both involving specific iron binding at the cell surface.

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.

The basis for evolution of DNA-binding specificity of the Aft1 transcription factor in yeasts.

The Saccharomyces cerevisiae Aft1 and Kluyveromyces lactis KlAft are orthologous yeast transcription activators that regulate the expression of the same group of iron-uptake genes but bind to the different DNA sites: TGCACCC for Aft1 and PuCACCC for KlAft. To establish whether the DNA-binding mechanisms of Aft1 and KlAft have diverged during the evolution of the Aft-type transcription factor, we examined the function of a nonconserved region in their DNA-binding domains. A large part of this region is composed of a sequence predicted to be disordered in structure and potentially phosphorylated. We show with deletion mutant analyses that this sequence is essential for the binding of Aft1 to its DNA site and for the iron uptake and growth of S. cerevisiae under iron-limited conditions. We constructed hybrid proteins by exchanging the nonconserved regions of Aft1 and KlAft. We show that the Aft1 region is necessary and sufficient for KlAft to bind efficiently to the Aft1 DNA site in S. cerevisiae and to complement the iron-dependent phenotype of the aft1Δaft2Δ mutant. This demonstrates that the changes in the nonconserved region of the Aft-type DNA-binding domain have led to changes in the DNA-binding specificity and have major consequences for the regulation of iron homeostasis. The combination of bioinformatic and experimental analyses indicates that the sequence TGCACCC is the most probable ancestral Aft-type element. Our findings suggest that the changes in the nonconserved region of the DNA-binding domain are responsible for the evolution of the TGCACCC sequence toward PuCACCC in the K. lactis species.

Gex1 is a yeast glutathione exchanger that interferes with pH and redox homeostasis.

In the yeast Saccharomyces cerevisiae, glutathione plays a major role in heavy metal detoxification and protection of cells against oxidative stress. We show that Gex1 is a new glutathione exchanger. Gex1 and its paralogue Gex2 belong to the major facilitator superfamily of transporters and display similarities to the Aft1-regulon family of siderophore transporters. Gex1 was found mostly at the vacuolar membrane and, to a lesser extent, at the plasma membrane. Gex1 expression was induced under conditions of iron depletion and was principally dependent on the iron-responsive transcription factor Aft2. However, a gex1Δ gex2Δ strain displayed no defect in known siderophore uptake. The deletion mutant accumulated intracellular glutathione, and cells overproducing Gex1 had low intracellular glutathione contents, with glutathione excreted into the extracellular medium. Furthermore, the strain overproducing Gex1 induced acidification of the cytosol, confirming the involvement of Gex1 in proton transport as a probable glutathione/proton antiporter. Finally, the imbalance of pH and glutathione homeostasis in the gex1Δ gex2Δ and Gex1-overproducing strains led to modulations of the cAMP/protein kinase A and protein kinase C1 mitogen-activated protein kinase signaling pathways.

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.

Siderophore uptake and use by the yeast Saccharomyces cerevisiae.

The non-reductive uptake of several siderophores (ferrioxamine B, ferrichrome, triacetylfusarinine C and ferricrocin) by various strains of Saccharomyces cerevisiae was studied. Several aspects of siderophore transport were examined, including specificity of transport, regulation of transport and intracellular localization of the ferri-siderophores. Ferrioxamine B was taken up preferentially via the products of the SIT1 gene and triacetylfusarinine C by the TAF1 gene product, but the specificity was not absolute. Ferrichrome and ferricrocin uptake was not dependent on a single major facilitator superfamily (MFS) gene product. The apparent specificity of transport was strongly dependent on the genetic background of the cells. Non-reductive uptake of siderophores was induced under more stringent conditions (of iron deprivation) than was the reductive uptake of ferric citrate. Regulation of transport depended on the transcriptional factors Aft1 and Tup1/Ssn6. Cells disrupted for the TUP1 or SSN6 genes were constitutively derepressed for the uptake of ferrichrome, ferricrocin or ferrioxamine B, but not for the uptake of triacetylfusarinine C. Cells bearing the AFT1(up) mutation accumulated large amounts of ferric siderophores. Intracellular decomplexation of the siderophores occurred when transcription of the AFT1(up) gene was repressed. Ferrioxamine B and ferrichrome seemed to accumulate in an endosomal compartment, as shown by biochemical studies and by confocal microscopy study of cells loaded with a fluorescent derivative of ferrichrome. Endocytosis was, however, not involved in the non-reductive uptake of siderophores.