TheyefM-yoeBToxin-Antitoxin Systems ofEscherichia coliandStreptococcus pneumoniae: Functional and Structural Correlation

Abstract

Toxin-antitoxin loci belonging to the yefM-yoeB family are located in the chromosome or in some plasmids of several bacteria. We cloned the yefM-yoeB locus of Streptococcus pneumoniae, and these genes encode bona fide antitoxin (YefM Spn ) and toxin (YoeB Spn ) products. We showed that overproduction of YoeB Spn is toxic to Escherichia coli cells, leading to severe inhibition of cell growth and to a reduction in cell viability; this toxicity was more pronounced in an E. coli B strain than in two E. coli K-12 strains. The YoeB Spn -mediated toxicity could be reversed by the cognate antitoxin, YefM Spn , but not by overproduction of the E. coli YefM antitoxin. The pneumococcal proteins were purified and were shown to interact with each other both in vitro and in vivo. Far-UV circular dichroism analyses indicated that the pneumococcal antitoxin was partially, but not totally, unfolded and was different than its E. coli counterpart. Molecular modeling showed that the toxins belonging to the family were homologous, whereas the antitoxins appeared to be specifically designed for each bacterial locus; thus, the toxin-antitoxin interactions were adapted to the different bacterial environmental conditions. Both structural features, folding and the molecular modeled structure, could explain the lack of crosscomplementation between the pneumococcal and E. coli antitoxins.The gram-positive, spherical bacterium Streptococcus pneumoniae (pneumococcus) is the cause of many human diseases, such as pneumonia, bacterial blood poisoning (bacteremia), inflammation of the membranes surrounding the brain and spinal cord (meningitis), middle-ear infection (otitis media), osteomyelitis, septic arthritis, endocarditis, peritonitis, pericarditis, and sinusitis; pneumonia is the most severe disease (15,28). Although the pneumococcus can normally be found in the noses and throats of healthy individuals, it can grow and cause infection when the immune system is weakened. The people who are most at risk of developing pneumococcal pneumonia have a weakened immune system. These people include the elderly, infants, cancer patients, AIDS patients, postoperative patients, alcoholics, and people with diabetes. The global rate of mortality is more than 1,000,000 people per year, and this figure represents about 15 to 20% of the people infected.

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

“…The reductions were about 8 logarithmic units for the B strain (BL21) and 4 to 5 logarithmic units for the other two K-12 strains (MG1655 and TOP10) after 6 h of exposure to YoeB (116). On the other hand, the induction of the entire yefM-yoeB locus was nontoxic to the cells.…”

“…The reduction in the number of cells that were able to form colonies following yoeB transcription could be alleviated by the later transcription of yefM: the numbers of CFU were reduced to 60% after 2 h and only 4% after 4 h of yoeB transcription, compared to 0.4% without YefM production, signifying that there is a window period for cell resuscitation (116). Moreover, the expression of yoeB in a Lon-deficient strain led to 85% of cells being able to form colonies after 1 h of yoeB overexpression, compared to only 25% of the colonies formed by the wild-type strain (116), indicating that Lon protease could play a role in YoeB-mediated toxicity. The target of pneumococcal YoeB has not been determined so far, but E. coli YoeB was shown to inhibit translation initiation by causing the cleavage of mRNAs at 3 bases downstream of the initiation codons in vivo (157).…”

“…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.…”

“…The chromosomal pezAT (pneumococcal epsilon-zeta antitoxin-toxin) genes were named after their homologue, the epsilon-zeta TAS, which was discovered in plasmid pSM19035 of Streptococcus pyogenes (18,20,82). Finally, the pneumococcal yefM-yoeB TAS showed homology to the E. coli counterpart (116,122) as well as to the axe-txe TAS of plasmid pRUM of Enterococcus faecium (65,116). These three functional pneumococcal TA pairs (relBE2, pezAT, and yefM-yoeB) are re-viewed below in terms of their genetic organizations, their transcriptional regulation, their functional activities, and, finally, the available details on their structures.…”

Characterization of the phd-doc and ccd Toxin-Antitoxin Cassettes from Vibrio Superintegrons

“…We had previously observed similar one-way interaction between the ccd F system and the chromosomal system found in E. coli O157:H7 (15), while Matsuba and colleagues had made a similar observation for the PemK toxin of the pem (parD) TA system located on the R100 (R1) plasmid of E. coli and the two antitoxins of its chromosomal homologues, which are unable to counteract the toxicity of the plasmid-encoded toxin (72), except when mutated (71). YefM-YoeB homologues carried either on plasmids or on chromosomes have also been found to poorly cross talk, at most (73,74). Thus, it appears that the plasmidic systems are "dominant" over their chromosomal homologues in many cases.…”