Table of contents

Volume 3, Issue 6, pp. 232 - 262, June 2016

Issue cover
Cover: Scanning electron microscopy image of E. coli undergoing programmed cell death triggered by ObgE*, a toxic isoform of the essential GTPase ObgE. Image by Liselot Dewachter, Natalie Verstraeten, Daniel Monteyne, David Pérez-Morga, Maarten Fauvart, and Jan Michiels (KU Leuven, Belgium); modified by MIC. The cover is published under the Creative Commons Attribution (CC BY) license. Enlarge issue cover

News and Thoughts

Non-genetic impact factors on chronological lifespan and stress resistance of baker’s yeast

Michael Sauer and Diethard Mattanovich

page 232-235 | 10.15698/mic2016.06.504 | Full text | PDF | Abstract

Survival under nutrient limitation is an essential feature of microbial cells, and it is defined by the chronological lifespan. We summarize recent findings, illustrating how crucial the choice of the experimental setup is for the interpretation of data in this field. Especially the impact of oxygen supply differs depending on the culture type, highlighting the differences of alternatives like the retentostat to classical batch cultures. Finally the importance of culture conditions on cell aging and survival in biotechnological processes is highlighted.

Research Articles

Construction and evaluation of yeast expression networks by database-guided predictions

Katharina Papsdorf, Siyuan Sima, Gerhard Richter, Klaus Richter

page 236-247 | 10.15698/mic2016.06.505 | Full text | PDF | Abstract

DNA-Microarrays are powerful tools to obtain expression data on the genome-wide scale. We performed microarray experiments to elucidate the transcriptional networks, which are up- or down-regulated in response to the expression of toxic polyglutamine proteins in yeast. Such experiments initially generate hit lists containing differentially expressed genes. To look into transcriptional responses, we constructed networks from these genes. We therefore developed an algorithm, which is capable of dealing with very small numbers of microarrays by clustering the hits based on co-regulatory relationships obtained from the SPELL database. Here, we evaluate this algorithm according to several criteria and further develop its statistical capabilities. Initially, we define how the number of SPELL-derived co-regulated genes and the number of input hits influences the quality of the networks. We then show the ability of our networks to accurately predict further differentially expressed genes. Including these predicted genes into the networks improves the network quality and allows quantifying the predictive strength of the networks based on a newly implemented scoring method. We find that this approach is useful for our own experimental data sets and also for many other data sets which we tested from the SPELL microarray database. Furthermore, the clusters obtained by the described algorithm greatly improve the assignment to biological processes and transcription factors for the individual clusters. Thus, the described clustering approach, which will be available through the ClusterEx web interface, and the evaluation parameters derived from it represent valuable tools for the fast and informative analysis of yeast microarray data.

Research Reports

Increased spontaneous recombination in RNase H2-deficient cells arises from multiple contiguous rNMPs and not from single rNMP residues incorporated by DNA polymerase epsilon

Anastasiya Epshtein, Catherine J. Potenski, and Hannah L. Klein

page 248-254 | 10.15698/mic2016.06.506 | Full text | PDF | Abstract

Ribonucleotides can become embedded in DNA from insertion by DNA polymerases, failure to remove Okazaki fragment primers, R-loops that can prime replication, and RNA/cDNA-mediated recombination. RNA:DNA hybrids are removed by RNase H enzymes. Single rNMPs in DNA are removed by RNase H2 and if they remain on the leading strand, can lead to mutagenesis in a Top1-dependent pathway. rNMPs in DNA can also stimulate genome instability, among which are homologous recombination gene conversion events. We previously found that, similar to the rNMP-stimulated mutagenesis, rNMP-stimulated recombination was also Top1-dependent. However, in contrast to mutagenesis, we report here that recombination is not stimulated by rNMPs incorporated by the replicative polymerase epsilon. Instead, recombination seems to be stimulated by multiple contiguous rNMPs, which may arise from R-loops or replication priming events.

Microreviews

The bacterial cell cycle checkpoint protein Obg and its role in programmed cell death

Liselot Dewachter, Natalie Verstraeten, Maarten Fauvart and Jan Michiels

page 255-256 | 10.15698/mic2016.06.507 | Full text | PDF | Abstract

The phenomenon of programmed cell death (PCD), in which cells initiate their own demise, is not restricted to multicellular organisms. Unicellular organisms, both eukaryotes and prokaryotes, also possess pathways that mediate PCD. We recently identified a PCD mechanism in Escherichia coli that is triggered by a mutant isoform of the essential GTPase ObgE (Obg of E. coli). Importantly, the PCD pathway mediated by mutant Obg (Obg*) differs fundamentally from other previously described bacterial PCD pathways and thus constitutes a new mode of PCD. ObgE was previously proposed to act as a cell cycle checkpoint protein able to halt cell division. The implication of ObgE in the regulation of PCD further increases the similarity between this protein and eukaryotic cell cycle regulators that are capable of doing both. Moreover, since Obg is conserved in eukaryotes, the elucidation of this cell death mechanism might contribute to the understanding of PCD in higher organisms. Additionally, if Obg*-mediated PCD is conserved among different bacterial species, it will be a prime target for the development of innovative antibacterials that artificially induce this pathway.

Bacterial outer membrane vesicle biogenesis: a new mechanism and its implications

Sandro Roier, Franz G. Zingl, Fatih Cakar, and Stefan Schild

page 257-259 | 10.15698/mic2016.06.508 | Full text | PDF | Abstract

Outer membrane vesicle (OMV) release by Gram-negative bacteria has been observed and studied for decades. First considered as a by-product of cell lysis, it soon became evident that OMVs are actively secreted from the outer membrane (OM) of Gram-negative bacteria. Accordingly, these small particles (~ 10-300 nm in diameter) consist mainly of OM components like phospholipids (PLs), OM proteins, and lipopolysaccharides or lipooligosaccharides. However, OMVs may also comprise periplasmic, inner membrane, or cytoplasmic components. Since the shedding of substantial amounts of OM material represents a significant energy cost to the bacterial cell, OMV production must have some vital biological functions for Gram-negative bacteria. Indeed, intense research on that topic revealed that OMVs play important roles in bacterial physiology and pathogenesis, ranging from secretion and delivery of biomolecules (for example, toxins, DNA, or quorum sensing molecules) over stress response and biofilm formation to immunomodulation and adherence to host cells. Only recently researchers have begun to elucidate the mechanistic aspects of OMV formation, but a general mechanism for the biogenesis of these vesicles is still lacking. Here we review the findings and implications of our recent study published in Nature Communications (Roier S, et al. (2016) Nat. Commun. 7:10515), where we propose a novel and highly conserved bacterial OMV biogenesis mechanism based on PL accumulation in the outer leaflet of the OM. This mechanism might not only have important pathophysiological roles in vivo, but also represents the first general mechanism of OMV formation applicable to all Gram-negative bacteria.

Metabolic network structure and function in bacteria goes beyond conserved enzyme components

Jannell V. Bazurto and Diana M. Downs

page 260-262 | 10.15698/mic2016.06.509 | Full text | PDF | Abstract

For decades, experimental work has laid the foundation for our understanding of the linear and branched pathways that are integrated to form the metabolic networks on which life is built. Genetic and biochemical approaches applied in model organisms generate empirical data that correlate genes, gene products and their biological activities. In the post-genomic era, these results have served as the basis for the genome annotation that is routinely used to infer the metabolic capabilities of an organism and mathematically model the presumed metabolic network structure. At large, genome annotation and metabolic network reconstructions have demystified genomic content of non-culturable microorganisms and allowed researchers to explore the breadth of metabolisms in silico. Mis-annotation aside, it is unclear whether in silico reconstructions of metabolic structure from component parts accurately captures the higher levels of network organization and flux distribution. For this approach to provide accurate predictions, one must assume that the conservation of metabolic components leads to conservation of metabolic network architecture and function. This assumption has not been rigorously tested. Here we describe the implications of a recent study (MBio 5;7(1): e01840-15), which demonstrated that conservation of metabolic components was not sufficient to predict network structure and function.