Abstract
Type II (proteic) toxin-antitoxin systems (TAS) are ubiquitous among bacteria. In the chromosome of the pathogenic bacterium Streptococcus pneumoniae, there are at least eight putative TAS, one of them being the yefM-yoeB(Spn) operon studied here. Through footprinting analyses, we showed that purified YefM(Spn) antitoxin and the YefM-YoeB(Spn) TA protein complex bind to a palindrome sequence encompassing the -35 region of the main promoter (P(yefM2)) of the operon. Thus, the locus appeared to be negatively autoregulated with respect to P(yefM2), since YefM(Spn) behaved as a weak repressor with YoeB(Spn) as a corepressor. Interestingly, a BOX element, composed of a single copy (each) of the boxA and boxC subelements, was found upstream of promoter P(yefM2). BOX sequences are pneumococcal, perhaps mobile, genetic elements that have been associated with bacterial processes such as phase variation, virulence regulation, and genetic competence. In the yefM-yoeB(Spn) locus, the boxAC element provided an additional weak promoter, P(yefM1), upstream of P(yefM2) which was not regulated by the TA proteins. In addition, transcriptional fusions with a lacZ reporter gene showed that P(yefM1) was constitutive albeit weaker than P(yefM2). Intriguingly, the coupling of the boxAC element to P(yefM1) and yefM(Spn) in cis (but not in trans) led to transcriptional activation, indicating that the regulation of the yefM-yoeB(Spn) locus differs somewhat from that of other TA loci and may involve as yet unidentified elements. Conservation of the boxAC sequences in all available sequenced genomes of S. pneumoniae which contained the yefM-yoeB(Spn) locus suggested that its presence may provide a selective advantage to the bacterium.
Highlights
IntroductionType II (proteic) toxin-antitoxin systems (TAS) are ubiquitous among bacteria. In the chromosome of the pathogenic bacterium Streptococcus pneumoniae, there are at least eight putative TAS, one of them being the yefM-yoeBSpn operon studied here
Type II toxin-antitoxin systems (TAS) are ubiquitous among bacteria
Sults showed that 7 strains that harbor solo YefMSpn antitoxin without its YoeBSpn toxin counterpart were all mutated or truncated at the 3Ј terminus, whereas no solo YoeBSpn homologues were identified
Summary
Type II (proteic) toxin-antitoxin systems (TAS) are ubiquitous among bacteria. In the chromosome of the pathogenic bacterium Streptococcus pneumoniae, there are at least eight putative TAS, one of them being the yefM-yoeBSpn operon studied here. In the yefM-yoeBSpn locus, the boxAC element provided an additional weak promoter, PyefM1, upstream of PyefM2 which was not regulated by the TA proteins. It would seem that at least in the case of mazEF, cell death is a population-dependent phenomenon requiring a quorum-sensing molecule, termed extracellular death factor, which is a linear pentapeptide (NNWNN) important for mazEF-mediated killing activity [18] Another line of independent investigations led to the different view that TAS would function as modulators of the global levels of translation during environmental stress and that the toxin-mediated inhibition of protein synthesis led to reversible cell stasis rather than cell death [4, 38]. An exception can be found in the case of the plasmid pSM19035 of Streptococus pyogenes, in which the expression of the epsilon-zeta (ε) TA operon is regulated by the global plasmid-encoded Omega () transcriptional repressor [22, 55]
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