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-yoeBSpn operon studied here. Through footprinting analyses, we showed that purified YefMSpn antitoxin and the YefM-YoeBSpn TA protein complex bind to a palindrome sequence encompassing the −35 region of the main promoter (PyefM2) of the operon. Thus, the locus appeared to be negatively autoregulated with respect to PyefM2, since YefMSpn behaved as a weak repressor with YoeBSpn as a corepressor. Interestingly, a BOX element, composed of a single copy (each) of the boxA and boxC subelements, was found upstream of promoter PyefM2. 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-yoeBSpn locus, the boxAC element provided an additional weak promoter, PyefM1, upstream of PyefM2 which was not regulated by the TA proteins. In addition, transcriptional fusions with a lacZ reporter gene showed that PyefM1 was constitutive albeit weaker than PyefM2. Intriguingly, the coupling of the boxAC element to PyefM1 and yefMSpn in cis (but not in trans) led to transcriptional activation, indicating that the regulation of the yefM-yoeBSpn 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-yoeBSpn locus suggested that its presence may provide a selective advantage to the bacterium