What is the benefit for E. coli to have multiple toxin-antitoxin systems in their genomes?

Laboratoire de Génétique des Procaryotes, Institut de Biologie et Médecine Moléculaires, Université Libre de Bruxelles, 12 rue des Professeurs Jeener et Brachet, B: 6041 Gosselies, Belgium

^* To whom correspondence should be addressed. Email: lvmelder{at}ulb.ac.be.

	Abstract

The E. coli K-12 chromosome encodes at least five proteic toxin-antitoxin (TA) systems. The mazEF and relBE systems have been extensively characterised 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 behaviour 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 in all these conditions, even in 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 in 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.