Chloramphenicol Resistance in Clostridium difficile Is Encoded on Tn 4453 Transposons That Are Closely Related to Tn 4451 from Clostridium perfringens

Abstract

The chloramphenicol resistance gene catD fromClostridium difficile was shown to be encoded on the transposons Tn4453a and Tn4453b, which were structurally and functionally related to Tn4451 fromClostridium perfringens. Tn4453a and Tn4453b excised precisely from recombinant plasmids, generating a circular form, as is the case for Tn4451. Evidence that this process is mediated by Tn4453-encodedtnpX genes was obtained from experiments which showed that in trans these genes complemented a Tn4451tnpXΔ1 mutation for excision. Nucleotide sequencing showed that the joint of the circular form generated by the excision of Tn4453a and Tn4453b was similar to that from Tn4451. These results suggest that the Tn4453-encoded TnpX proteins bind to similar DNA target sequences and function in a manner comparable to that of TnpX from Tn4451. Furthermore, it has been shown that Tn4453a and Tn4453b can be transferred to suitable recipient cells by RP4 and therefore are mobilizable transposons. It is concluded that, like Tn4451, they must encode a functional tnpZ gene and a targetoriT or RSA site. The finding that related transposable elements are present in C. difficile andC. perfringens has implications for the evolution and dissemination of antibiotic resistance genes and the mobile elements on which they are found within the clostridia.

The clostridial mobilisable transposons

“…For example, pIP401 is indistinguishable from pCW3 except that it carries the chloramphenicol resistance transposon Tn4451 [9]. A very similar transposon, Tn4453, is found in C. difficile [10] but it is located on the chromosome. C. difficile also carries the conjugative tetracycline resistance transposon Tn5397.…”

“…One other chloramphenicol-resistant C. perfringens 2034 V. Adams et al Clostridial mobilisable transposons strain has been identified; it carries a chromosomal catQ gene [16]. Chloramphenicol resistance in C. difficile is also chromosomally encoded but by a catP gene that is located on two mobile elements very similar to Tn4451, known as Tn4453a and Tn4453b [10]. These elements belong to a family of clostridial mobilisable transposons known as the Tn4451/53 family.…”

“…TnpZ-mediated mobilisation occurs from an E. coli donor to both E. coli or C. perfringens recipients but is dependent upon the presence of an RP4-derivative in the donor [19]. The process occurs with Tn4451, Tn4453a and Tn4453b, although the mobilisation frequencies vary [10]. The Tn4451 family of elements therefore are clearly defined as mobilisable transposons that in the presence of an appropriate conjugative plasmid can promote the mobilisation of nonconjugative plasmids on which they reside.…”

“…The significance of these findings is unknown, but the results suggest that TnpV and TnpY may have the ability to bind DNA. Two other members of this transposon family, Tn4453a and Tn4453b, were identified from the chromosome of a single chloramphenicol-resistant C. difficile isolate [10]. Tn4453a has been completely sequenced and has 89% nucleotide sequence identity to Tn4451 [31].…”

Clostridium perfringens type A–E toxin plasmids

“…This Rep protein has a predicted pI of 10, consistent with this being a DNA-binding protein [122, 129, 142]. The Rep protein of C. perfringens plasmids does not share similarity, motifs or domains with Rep proteins of other species, which probably explains why the C. perfringens plasmids have not been observed in other clostridial species or bacterial genera [143, 144]. …”