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Volume 5, Issue 10, pp. 424 - 471, October 2018

Issue cover
Cover: Cartoon illustration of a model presented by Sewell, Han and Qi (this issue). The “blue river” illustrates the intestine of a host animal, where commensal E. coli (green character) secrete enterobactin (Ent, yellow triangle) and use it to fetch iron (red ball). The host cell (“fisherman”) can also use Ent to obtain iron through the interaction with ATP synthase alpha subunit (ATPS-alpha, fishing pole). Concept by Bin Qi, Aileen Sewell and Min Han (University of Colorado Boulder, USA); art created by David Deen (USA); image modified by MIC. The cover is published under the Creative Commons Attribution (CC BY) license. Enlarge issue cover


The biosynthesis of pyoverdines

Michael T. Ringel and Thomas Brüser

page 424-437 | 10.15698/mic2018.10.649 | Full text | PDF | Abstract

Pyoverdines are fluorescent siderophores of pseudomonads that play important roles for growth under iron-limiting conditions. The production of pyoverdines by fluorescent pseudomonads permits their colonization of hosts ranging from humans to plants. Prominent examples include pathogenic or non-pathogenic species such as Pseudomonas aeruginosa, P. putida, P. syringae, or P. fluorescens. Many distinct pyoverdines have been identified, all of which have a dihydroxyquinoline fluorophore in common, derived from oxidative cyclizations of non-ribosomal peptides. These serve as precursor of pyoverdines and are commonly known as ferribactins. Ferribactins of distinct species or even strains often differ in their sequence, resulting in a large variety of pyoverdines. However, synthesis of all ferribactins begins with an L-Glu/D-Tyr/L-Dab sequence, and the fluorophore is generated from the D-Tyr/L-Dab residues. In addition, the initial L-Glu residue is modified to various acids and amides that are responsible for the range of distinguishable pyoverdines in individual strains. While ferribactin synthesis is a cytoplasmic process, the maturation to the fluorescent pyoverdine as well as the tailoring of the initial glutamate are exclusively periplasmic processes that have been a mystery until recently. Here we review the current knowledge of pyoverdine biosynthesis with a focus on the recent advancements regarding the periplasmic maturation and tailoring reactions.

A Cinderella story: how the vacuolar proteases Pep4 and Prb1 do more than cleaning up the cell’s mass degradation processes

Winnie Kerstens and Patrick Van Dijck

page 438-443 | 10.15698/mic2018.10.650 | Full text | PDF | Abstract

Recently, several research groups have assigned non-vacuolar functions to the well-known Saccharomyces cerevisiae vacuolar proteases Pep4 and Prb1, which are also known as proteinases A and B. These non-vacuolar activities seem to be autophagy-independent and stress-induced and suggest an unexplored but possibly prominent role for the proteases outside the vacuole. The functions range from the involvement in programmed cell death, to protection from hazardous protein forms and regulation of gene expression. We propose that a deeper understanding of these molecular processes will provide new insights that will be important for both fungal biology as well as studies in mammalian cells, as they might open up perspectives in the search for novel drug targets. To illustrate this, we summarize the recent literature on non-vacuolar Pep4 and Prb1 functions in S. cerevisiae and review the current data on the protein homologs in pathogenic fungi.

Research Articles

Trehalose-6-phosphate promotes fermentation and glucose repression in Saccharomyces cerevisiae

Rebeca L. Vicente, Lucie Spina, Jose P.L. Gómez, Sebastien Dejean, Jean-Luc Parrou and Jean Marie François

page 444-459 | 10.15698/mic2018.10.651 | Full text | PDF | Abstract

The yeast trehalose-6-phosphate synthase (Tps1) catalyzes the formation of trehalose-6-phosphate (T6P) in trehalose synthesis. Besides, Tps1 plays a key role in carbon and energy homeostasis in this microbial cell, as shown by the well documented loss of ATP and hyper accumulation of sugar phosphates in response to glucose addition in a mutant defective in this protein. The inability of a Saccharomyces cerevisiae tps1 mutant to cope with fermentable sugars is still a matter of debate. We reexamined this question through a quantitative analysis of the capability of TPS1 homologues from different origins to complement phenotypic defects of this mutant. Our results allowed to classify this complementation in three groups. A first group enclosed TPS1 of Klyveromyces lactis with that of S. cerevisiae as their expression in Sctps1 cells fully recovered wild type metabolic patterns and fermentation capacity in response to glucose. At the opposite was the group with TPS1 homologues from the bacteria Escherichia coli and Ralstonia solanacearum, the plant Arabidopsis thaliana and the insect Drosophila melanogaster whose metabolic profiles were comparable to those of a tps1 mutant, notably with almost no accumulation of T6P, strong impairment of ATP recovery and potent reduction of fermentation capacity, albeit these homologous genes were able to rescue growth of Sctps1 on glucose. In between was a group consisting of TPS1 homologues from other yeast species and filamentous fungi characterized by 5 to 10 times lower accumulation of T6P, a weaker recovery of ATP and a 3-times lower fermentation capacity than wild type. Finally, we found that glucose repression of gluconeogenic genes was strongly dependent on T6P. Altogether, our results suggest that the TPS protein is indispensable for growth on fermentable sugars, and points to a critical role of T6P as a sensing molecule that promotes sugar fermentation and glucose repression.

Research Reports

The translationally controlled tumor protein TCTP is involved in cell cycle progression and heat stress response in the bloodstream form of Trypanosoma brucei

Borka Jojic, Simona Amodeo and Torsten Ochsenreiter

page 460-468 | 10.15698/mic2018.10.652 | Full text | PDF | Abstract

The translationally controlled tumor protein TCTP, is a universally conserved protein that seems to be of essential function in all systems tested so far. TCTP is involved in a multitude of cellular functions including cell cycle control, cell division, apoptosis and many more. The mechanism of how TCTP is involved in most of these functions remains elusive. Here we describe that TCTP is a cytoplasmic protein involved in cell cycle regulation and heat stress response in the bloodstream form of Trypanosoma brucei.


An unexpected benefit from E. coli: how enterobactin benefits host health

Aileen K. Sewell, Min Han and Bin Qi

page 469-471 | 10.15698/mic2018.10.653 | Full text | PDF | Abstract

Iron plays many critical roles in human biology, such as aiding the transport of oxygen and mediating redox reactions. Iron is essential for life, yet little is known about how iron is taken up into mitochondria to impact the labile iron pool. Iron deficiency is one of the most prevalent human nutrient-deficiency diseases in the world and is a major cause of anemia that affects >25% of the world’s population, but unfortunately the current treatment (oral iron supplementation) is inefficient and has many side effects. A greater understanding of iron uptake, and discovery of molecules that aid in this process, may lead to more effective treatments for iron deficiency. In this study, we uncovered a unique and surprising role for an Escherichia coli-produced siderophore enterobactin (Ent) that facilitates iron uptake by the host, observed in both C. elegans and mammalian cells. Although siderophores are well-known Fe+3 scavengers, this activity has previously been described to only benefit iron acquisition by bacteria, not the host. This unexpected function is dependent on the binding of Ent to the host’s ATP synthase α-subunit but is independent of other subunits of the ATP synthase. This finding marks a major shift regarding the role of this siderophore in the “iron tug-of-war” paradigm, which is often used to describe the fight between the bacteria and the host for this essential micronutrient. Instead, this study presents E. coli as a commensal “friend” that provides a molecule that supports the host’s iron homeostasis. This work reveals a novel, beneficial role of a bacteria-generated molecule in aiding the host’s iron homeostasis, and points to surprising new benefits from commensal bacteria.

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