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Abstract
Streptococcus agalactiae or Group B Streptococcus (GBS) are opportunistic bacteria that can cause lethal sepsis in children and immuno-compromised patients. Their genome is a reservoir of mobile genetic elements that can be horizontally transferred. Among them, integrative and conjugative elements (ICEs) and the smaller integrative and mobilizable elements (IMEs) primarily reside in the bacterial chromosome, yet have the ability to be transferred between cells by conjugation. ICEs and IMEs are therefore a source of genetic variability that participates in the spread of antibiotic resistance. Although IMEs seem to be the most prevalent class of elements transferable by conjugation, they are poorly known. Here, we have studied a GBS-IME, termed IMESag-rpsI, which is widely distributed in GBS despite not carrying any apparent virulence trait. Analyses of 240 whole genomes showed that IMESag-rpsI is present in approximately 47% of the genomes, has a roughly constant size (approx. 9 kb) and is always integrated at a single location, the 3′-end of the gene encoding the ribosomal protein S9 (rpsI). Based on their genetic variation, several IMESag-rpsI types were defined (A–J) and classified in clonal complexes (CCs). CC1 was the most populated by IMESag-rpsI (more than 95%), mostly of type-A (71%). One CC1 strain (S. agalactiae HRC) was deep-sequenced to understand the rationale underlying type-A IMESag-rpsI enrichment in GBS. Thirteen open reading frames were identified, one of them encoding a protein (MobSag) belonging to the broadly distributed family of relaxases MOBV1. Protein MobSag was purified and, by a newly developed method, shown to cleave DNA at a specific dinucleotide. The S. agalactiae HRC-IMESag-rpsI is able to excise from the chromosome, as shown by the presence of circular intermediates, and it harbours a fully functional mobilization module. Further, the mobSag gene encoded by this mobile element is able to promote plasmid transfer among pneumococcal strains, suggesting that MobSag facilitates the spread of IMESag-rpsI and that this spread would explain the presence of the same IMESag-rpsI type in GBS strains belonging to different CCs.
Highlights
Conjugation is the major process involved in horizontal gene transfer (HGT) and is responsible for the spreading of antibiotic resistance and virulence traits among bacterial populations
MOBV1 relaxases are overrepresented in mobilizable elements from Firmicutes, a clade including genera of outstanding relevance in the spread of antibiotic resistance traits, such as Staphylococcus, Enterococcus or Streptococcus [27]
We reasoned that a global study of the distribution and the diversity of IMESag-rpsI, mediated by the MOBV1 relaxase it encodes, could help us to understand the dissemination of this particular integrative and mobilizable elements (IMEs)
Summary
Conjugation is the major process involved in horizontal gene transfer (HGT) and is responsible for the spreading of antibiotic resistance and virulence traits among bacterial populations. License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted use, provided the original author and source are credited. Transfer of a conjugative genetic element is initiated by an element-encoded protein, termed relaxase. Relaxases are site-specific endonucleases that cleave the DNA to be transferred at a specific region, the origin of transfer (oriT). Cleavage is exerted at the phosphodiester bond of a specific dinucleotide in one of the DNA strands. The relaxase-mediated nucleophilic attack generates a stable DNA–protein complex that blocks the 50-end and leaves a free 30-OH end in the DNA to be transferred (t-DNA). The t-DNA : relaxase complex is actively pumped to the recipient cell by the coupling protein and the type IV secretion system (T4SS). The t-DNA in the recipient cell replicates, most probably, through lagging-strand synthesis [3]
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