Characterization of a Small Mobilizable Transposon, MTnSag1, inStreptococcus agalactiae

Abstract

Group B streptococcus or Streptococcus agalactiae is a component of the normal flora of human mucosa and a well-known cause of invasive infection in neonates, pregnant women, and older individuals with underlying chronic illness (4, 11). Dissemination of mobile elements largely contributes to the increasing resistance to macrolides among streptococci. Mobilizable transposons are genetic elements that are smaller than conjugative transposons, can range in size from 4.7 kb (Tn5520) to 12.7 kb (Tn4555), and have been identified only in Bacteroides spp. and Clostridium spp. (2,14,16,17).In this report, we demonstrate that MTnSag1, a 1,724-bplong element that we previously identified in the clinical strain S. agalactiae UCN36 (accession number AY928180), is a new transposon that can be mobilized by the conjugative transposon Tn916 to S. agalactiae recipients (1).Sequence analysis of the MTnSag1 transposase gene. The MTnSag1 element contained two open reading frames in the same orientation, ORF1 and ORF2, with sizes of 1,038 and 495 bp, respectively. ORF2, called lnu(C), encoded a lincosamide O-nucleotidyltransferase conferring resistance to lincomycin (1). ORF1 encoded a putative protein related to the protein InsA of the insertion sequence IS1 (35% identity) and to several transposases described for Clostridium spp. (33 to 42% identity). MTnSag1 had a pair of 25-bp imperfect inverted repeats at its termini. A search for motifs and domains using the NCBI Blastp and EMBOSS helix-turn-helix programs (http://www.ncbi.nlm.nih.gov/ and http://www.bioweb.pasteur .fr) revealed the presence of potential zinc finger (ZF) and helix-Turn-helix (HTH) motifs in the N-terminal region of the MTnSag1 transposase that are characteristics of IS1 transposases (8, 15) (Fig. 1). The conserved C residues were at positions 52, 55, 75, and 78. Other conserved residues, including an aromatic amino acid (F at position 82 corresponding to W at position 39 for IS1A), were identified. A putative DDE motif was also evidenced in the C terminus of the MTnSag1 transposase. This DDE motif (D-67-D-92-E) was distantly related to the DDE motif of the IS1 members (D-56/80-D-21/ 24-E) but was in agreement with the consensus motif of known transposases and retroviral integrases (D-50/80-D33/138-E) (15). These results lead us to classify the MTnSag1 putative transposase in the IS1 family.Transferability of MTnSag1. The transferability of the MTnSag1 transposon was tested using filter mating with S. agalactiae BM132 (resistant to rifampin and fusidic acid) or BM134 (resistant to streptomycin), Enterococcus faecalis JH2-2, and Escherichia coli K-12 AG100A (1) as recipient strains. MTnSag1 was transferable from S. agalactiae UCN36 to S. agalactiae BM134 or BM132 at a frequency of (1.6 Ϯ 0.3) ϫ 10 Ϫ7 transconjugants per donor cell. No transfer to E. faecalis JH2-2 and E. coli K-12 AG100A was detected (Յ10 Ϫ9 transconjugants). Southern blot experiments revealed that six copies of MTnSag1 were present in S. agalactiae UCN36 but only one copy was present in three ...

Physiological impact of transposable elements encoding DDE transposases in the environmental adaptation of Streptococcus agalactiae

“…A circular form of MTnSag1 (tISSag10) was detected, but only in the surrogate host E. coli, and an origin of transfer (oriT) was located at the 39end of the lnuC gene. It was proposed that MTnSag1 (tISSag10) can be transferred by conjugation and that its acquisition by S. agalactiae might be efficient for the spread of lincomycin resistance since the majority of S. agalactiae strains possess Tn916-like genetic elements (Achard & Leclercq, 2007). However, the insertion of MTnSag1 (tISSag10) in the capsular operon of strain UCN36 has interrupted the cpsE gene; as a consequence the strain was non-typable (Achard et al, 2005).…”

“…tISSag10 carries the passenger gene (lnuC), which encodes a lincosamide O-nucleotidyltransferase conferring resistance to lincomycin (Achard & Leclercq, 2007). MTnSag1 (tISSag10) was also recently identified in the chromosome of the Streptococcus anginosus clinical isolate UCN93 (Gravey et al, 2013).…”

“…Some of the accessory genes present on these transposable elements were found to confer to the genome of the organism some environmental adaptive advantage, such as a new metabolic property, a resistance to pollutants or antibiotics, or an increase of pathogenicity (Achard & Leclercq, 2007;Casacuberta & González, 2013). Transposable elements can also act by modifying the expression of neighbouring genes at the integration site, or by promoting huge genomic rearrangements when they are substrates for homologous recombination (Casacuberta & González, 2013;Mahillon & Chandler, 1998;Maruyama et al, 2009).…”

“…The conjugative transposon Tn916 is one of the best characterized ICEs. About 80 % of human S. agalactiae isolates are resistant to tetracycline due to the presence of Tn916 or related ICEs (Achard & Leclercq, 2007;Le Bouguénec et al, 1990;Poyart et al, 2003). However, with the exception of the TnGBS conjugative transposons described below, transposition of Tn916 and of all S. agalactiae ICEs described so far is not mediated by a DDE transposase, but by a serine recombinase, such as Tn5397 from Clostridium difficile or a tyrosine recombinase such as Tn916 (Brochet et al, 2008a;Chuzeville et al, 2012;Da Cunha et al, 2013;Haenni et al, 2010;Puymège et al, 2013;Rajeev et al, 2009;Wang et al, 2006).…”

Conjugative Transfer and cis-Mobilization of a Genomic Island by an Integrative and Conjugative Element of Streptococcus agalactiae

“…Furthermore, Tn916 was found to mobilize in trans MTnSag1, an element from S agalactiae which encodes a DDE transposase and carries its own oriT unrelated to the one of Tn916 but lacks conjugation or mobilization genes (46). Mobile genomic islands (MGI) carrying their own tyrosine integrase and an their own oriT but no conjugation or mobilization gene were also reported to be mobilized in trans by an ICE harboring a related oriT in Vibrio (47).…”