Transposon Tn21, Flagship of the Floating Genome

Abstract

SUMMARY The transposon Tn21 and a group of closely related transposons (the Tn21 family) are involved in the global dissemination of antibiotic resistance determinants in gram-negative facultative bacteria. The molecular basis for their involvement is carriage by the Tn21 family of a mobile DNA element (the integron) encoding a site-specific system for the acquisition of multiple antibiotic resistance genes. The paradigm example, Tn21, also carries genes for its own transposition and a mercury resistance (mer) operon. We have compiled the entire 19,671-bp sequence of Tn21 and assessed the possible origins and functions of the genes it contains. Our assessment adds molecular detail to previous models of the evolution of Tn21 and is consistent with the insertion of the integron In2 into an ancestral Tn501-like mer transposon. Codon usage analysis indicates distinct host origins for the ancestral mer operon, the integron, and the gene cassette and two insertion sequences which lie within the integron. The sole gene of unknown function in the integron, orf5, resembles a puromycin-modifying enzyme from an antibiotic producing bacterium. A possible seventh gene in the mer operon (merE), perhaps with a role in Hg(II) transport, lies in the junction between the integron and the mer operon. Analysis of the region interrupted by insertion of the integron suggests that the putative transposition regulator, tnpM, is the C-terminal vestige of a tyrosine kinase sensor present in the ancestral mer transposon. The extensive dissemination of the Tn21 family may have resulted from the fortuitous association of a genetic element for accumulating multiple antibiotic resistances (the integron) with one conferring resistance to a toxic metal at a time when clinical, agricultural, and industrial practices were rapidly increasing the exposure to both types of selective agents. The compendium offered here will provide a reference point for ongoing observations of related elements in multiply resistant strains emerging worldwide.

The soil resistome: a critical review on antibiotic resistance origins, ecology and dissemination potential in telluric bacteria

“…The soil resistome 923 (Liebert et al, 1999;Roberts and Mullany, 2009) to hypermutator phenotypes (Turrientes et al, 2010). Each of these different mechanisms not only have implications at different population levels and have all been shown to mediate antibiotic resistance gene transfer by mobilizing resistance genes carried on their accessory module, but are also induced by exposure to antibiotic molecules (Jolivet-Gougeon et al, 2011;Baharoglu et al, 2012;Blázquez et al, 2012).…”

Mercuric resistance genes in gram-positive oral bacteria

“…The predominant mechanism of mercury resistance in bacteria is the enzymatic reduction of mercury, Hg 2þ to Hg 0 by mercuric reductase enzyme. The mercuric reductase is encoded by the merA gene, which usually forms part of a mer operon consisting of genes coding in addition for proteins involved in mercury transport (MerC, MerE, MerF, MerP and MerT) [1][2][3], the regulation of mer gene expression (MerR and MerD) [4,5], and in some cases mercuric lyase enzymes conferring in combination with MerA, organomercuric resistance (MerB1, MerB3, and MerB3 in Bacillus cereus RC607) [6,7]. For comprehensive reviews on mercury resistance see [8][9][10][11].…”

Detection and characterisation of integrons in Salmonella enterica serotype Enteritidis

“…Bacterial resistance to an increasing number of antimicrobial agents is a well-established problem. Several mechanisms involving mobile genetic elements, such as plasmids and transposons, have been shown to contribute to the spread of this resistance [1,2]. In recent years, a novel group of DNA elements able to incorporate antibiotic resistance genes by a site-speci¢c recombination have been identi¢ed in Gram-negative bacteria.…”