RESUMO
The Escherichia coli K-12 chromosome encodes at least five proteic toxin-antitoxin (TA) systems. The mazEF and relBE systems have been extensively characterized and were proposed to be general stress response modules. On one hand, mazEF was proposed to act as a programmed cell death system that is triggered by a variety of stresses. On the other hand, relBE and mazEF were proposed to serve as growth modulators that induce a dormancy state during amino acid starvation. These conflicting hypotheses led us to test a possible synergetic effect of the five characterized E. coli TA systems on stress response. We compared the behavior of a wild-type strain and its derivative devoid of the five TA systems under various stress conditions. We were unable to detect TA-dependent programmed cell death under any of these conditions, even under conditions previously reported to induce it. Thus, our results rule out the programmed-cell-death hypothesis. Moreover, the presence of the five TA systems advantaged neither recovery from the different stresses nor cell growth under nutrient-limited conditions in competition experiments. This casts a doubt on whether TA systems significantly influence bacterial fitness and competitiveness during non-steady-state growth conditions.
Assuntos
Adaptação Fisiológica , Toxinas Bacterianas/toxicidade , Proteínas de Escherichia coli/fisiologia , Escherichia coli/crescimento & desenvolvimento , Viabilidade Microbiana , Ácidos/farmacologia , Aminoácidos/metabolismo , Antibacterianos/farmacologia , Apoptose , Toxinas Bacterianas/genética , Contagem de Colônia Microbiana , Proteínas de Ligação a DNA/genética , Proteínas de Ligação a DNA/fisiologia , Endorribonucleases/genética , Endorribonucleases/fisiologia , Escherichia coli/genética , Escherichia coli/fisiologia , Proteínas de Escherichia coli/genética , Deleção de Genes , Rifampina/farmacologiaRESUMO
The Lon ATP-dependent protease plays a major role in protein quality control. An increasing number of regulatory proteins, however, are being identified as Lon substrates, thus indicating that in addition to its housekeeping function, Lon plays an important role in regulating many biological processes in bacteria. This review presents and discusses the involvement of Lon in different aspects of bacterial physiology, including cell differentiation, sporulation, pathogenicity and survival under starvation conditions.
Assuntos
Fenômenos Fisiológicos Bacterianos , Proteínas de Bactérias/fisiologia , Protease La/fisiologia , Bacillus subtilis/enzimologia , Diferenciação Celular , Divisão Celular , Metilação de DNA , Ativação Enzimática , Escherichia coli/enzimologia , Regulação Bacteriana da Expressão Gênica , Regulação Enzimológica da Expressão Gênica , Salmonella typhimurium/enzimologia , Transdução de Sinais , Esporos Bacterianos/enzimologia , Especificidade por SubstratoRESUMO
The transposable and temperate phage Mu infects Escherichia coli where it can enter the lytic life-cycle or reside as a repressed and integrated prophage. The repressor protein Rep is the key element in the lysis-lysogeny decision. We have analyzed the fate of Rep in different mutants by Western blotting under two conditions that can induce a lysogen: high temperature and stationary phase. We show that, unexpectedly, Rep accumulates under all conditions where the prophage is completely derepressed, and that this accumulation is ClpX-dependent. An analysis of the degradation kinetics shows that Rep is a target of two protease systems: inactivation of either the clpP or lon gene results in a stabilization of Rep. Such a reaction scheme explains the counterintuitive observation that derepression is correlated with high repressor concentration. We conclude that under all conditions of phage induction the repressor is sequestered in a non-active form. A quantitative simulation accounts for our experimental data. It provides a model that captures the essential features of Mu induction and explains some of the mechanisms by which the physiological signals affecting the lysis-lysogeny decision converge onto Rep.
Assuntos
Bacteriófago mu/genética , Bacteriófago mu/fisiologia , Regulação Viral da Expressão Gênica , Lisogenia/genética , Imunoprecipitação da Cromatina , Modelos Genéticos , Mutação/genética , Proteínas Repressoras/genética , Proteínas Repressoras/metabolismo , Temperatura , Proteínas Virais/genética , Proteínas Virais/metabolismoRESUMO
In Escherichia coli, the Lon ATP-dependent protease is responsible for degradation of several regulatory proteins and for the elimination of abnormal proteins. Previous studies have shown that the overproduction of Lon is lethal. Here, we showed that Lon overproduction specifically inhibits translation through at least two different pathways. We have identified one of the pathways as being the chromosomal yefM-yoeB toxin-antitoxin system. The existence of a second pathway is demonstrated by the observation that the deletion of the yefM-yoeB system did not completely suppress lethality and translation inhibition. We also showed that the YoeB toxin induces cleavage of translated mRNAs and that Lon overproduction specifically activates YoeB-dependent mRNAs cleavage. Indeed, none of the other identified chromosomal toxin-antitoxin systems (relBE, mazEF, chpB and dinJ-yafQ) was involved in Lon-dependent lethality, translation inhibition and mRNA cleavage even though the RelB and MazE antitoxins are known to be Lon substrates. Based on our results and other studies, translation inhibition appears to be the key element that triggers chromosomal toxin-antitoxin systems. We propose that under Lon overproduction conditions, translation inhibition is mediated by Lon degradation of a component of the YoeB-independent pathway, in turn activating the YoeB toxin by preventing synthesis of its unstable YefM antidote.
