Genetic Regulation of the yefM-yoeB Toxin-Antitoxin Locus of Streptococcus pneumoniae

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.

Toxin-Antitoxin Genes of the Gram-Positive Pathogen Streptococcus pneumoniae: So Few and Yet So Many

“…The influence of the BOX subelement boxAC on the expression of the yefM-yoeB TAS of S. pneumoniae was reported recently by our laboratory (21); this boxAC is highly conserved in all the S. pneumoniae strains that harbor the yefM-yoeB TAS. In the strains that we studied here, BOX elements were annotated in only two instances, R6 and ATCC 700669.…”

“…Instead, transcriptional fusion assays indicated that the presence of boxAC (P yefM1 ) together with P yefM2 and the yefM reading frame in cis led to transcriptional activation in E. coli. It was suggested that activation could be due to still unknown host binding factors and/or other cis-acting elements (21). The presence of this boxAC element is universal, as it is found in all sequenced pneumococcal strains that harbor an intact yefM-yoeB locus but is absent in other yefM-yoeB homologues of different bacteria.…”

“…The pneumococcal yefM-yoeB TA pair was initially proposed to be designated relBE3 due to its low-level but significant similarity to the E. coli relBE system (58); however, we preferred to name it yefM-yoeB in previous work (21,116) and in this review. In addition to the low level of similarity between the gene products (38% between the E. coli RelB and the pneumococcal YefM proteins and 33% for the E. coli RelE and the pneumococcal YoeB proteins), differences also exist in the promoter regions.…”

“…indrome sequence that overlaps the Ϫ35 sequence of the promoter P yefM2 , whereas the YoeB toxin serves as a corepressor to further repress transcription (21). The incorporation of the boxAC element upstream of P yefM2 created an additional promoter, designated P yefM1 , which is about 15-fold weaker than P yefM2 but not regulated by YefM and YoeB (Fig.…”

“…In this review, we focus on the TA genes of S. pneumoniae because they constitute a fairly unexplored world: only three pneumococcal TA pairs, namely, RelBE2, PezAT, and YefMYoeB, have been studied so far, and only the PezAT TA pair has been characterized in terms of structure and function (21,82,113,117). Surprisingly enough, an in-depth study of possible TAS present in the genomes of pneumococcal strains that have been sequenced showed that their number is far larger than envisaged (90), and our results presented here show that their number can be even larger.…”