Interactions of the Streptococcus pneumoniae Toxin-Antitoxin RelBE Proteins with Their Target DNA

Inmaculada Moreno-Córdoba,Concha Nieto,Manuel Espinosa,Wai-Ting Chan

doi:10.3390/microorganisms9040851

Inmaculada Moreno-Córdoba, Concha Nieto + Show 2 more

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https://doi.org/10.3390/microorganisms9040851

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Abstract

Type II bacterial toxin-antitoxin (TA) systems are found in most bacteria, archaea, and mobile genetic elements. TAs are usually found as a bi-cistronic operon composed of an unstable antitoxin and a stable toxin that targets crucial cellular functions like DNA supercoiling, cell-wall synthesis or mRNA translation. The type II RelBE system encoded by the pathogen Streptococcus pneumoniae is highly conserved among different strains and participates in biofilm formation and response to oxidative stress. Here, we have analyzed the participation of the RelB antitoxin and the RelB:RelE protein complex in the self-regulation of the pneumococcal relBE operon. RelB acted as a weak repressor, whereas RelE performed the role of a co-repressor. By DNA footprinting experiments, we show that the proteins bind to a region that encompasses two palindromic sequences that are located around the −10 sequences of the single promoter that directs the synthesis of the relBE mRNA. High-resolution footprinting assays showed the distribution of bases whose deoxyriboses are protected by the bound proteins, demonstrating that RelB and RelB:RelE contacted the DNA backbone on one face of the DNA helix and that these interactions extended beyond the palindromic sequences. Our findings suggest that the binding of the RelBE proteins to its DNA target would lead to direct inhibition of the binding of the host RNA polymerase to the relBE promoter.

Highlights

Chromosomally-encoded Type II Toxin-Antitoxin bacterial systems (TAs) are constituted by a pair of genes organized as an operon in which the gene encoding the antitoxin usually precedes the gene encoding its cognate toxin [1,2,3]
TAs are ubiquitous in the bacterial world, and they may be present in multiple copies in the bacterial chromosome, posing the question of why there is a need for such genetic redundancy [10]
The pneumococcal relBE operon is present in all the strains sequenced so far, indicative of the relevance of this TA pair in the bacterial lifestyle and despite the variability of the genetic context of the operon in the different strains analyzed [38]

Summary

Introduction

Chromosomally-encoded Type II Toxin-Antitoxin bacterial systems (TAs) are constituted by a pair of genes organized as an operon in which the gene encoding the antitoxin usually precedes the gene encoding its cognate toxin [1,2,3]. Toxin activeness usually brings about a cell response that leads to a severe reduction in the metabolism, and as a consequence, a dormant state known as persistence [9]. A selfish qualification of these bacterial operons could lead to some of the adduced roles of TAs, permitting the chromosomal TAs to endure without providing any substantial advantage to their hosts [14].

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