TABLE 2.

Major AMR-encoding transposons^a

Tn (accession no.)	AMR locus and mechanism	Key genetic features and architecture	Reference
Tn6194 (HG475346)b	ermB: methylation of 23S rRNA of bacterial 50S ribosomal subunit, thereby reducing binding affinity of MLSB class antimicrobials	28,014 bp in size and 35 CDSs	44
		1 copy of ermB, distinguishing it from Tn5398, which has 2 copiesc
		Recombination module comprising int (1,446 bp) and xis (258 bp) genes
		int/xis genes invariably found adjacent to a tRNA gene (Arg)
		Contains toxin (ζ [159 bp]), antitoxin (ε [273 bp]), and 3′ cell surface protein (3,045 bp) genes
Tn6190 (FN665653)	tetM: mimics ribosomal elongation factors, protecting against antitranslational activity of tetracyclines	18,032 bp in sized	41
		Contains a recombination module comprising int (1,218 bp) and xis (204 bp) genes, distinguishing it from Tn5397, which contains a site-specific recombinase gene, tndXe
		orf12 (encoding a leader peptide) is absent and lacks the 1,831-bp group II intron in orf14, both characteristic features of Tn5397
Tn6164 (FN665653)f	tet-44: mimics ribosomal elongation factors, protecting them from antitranslational activity of tetracyclines	106,711 bp, containing 5 distinct modules (A to E) and 90 ORFs originating from diverse bacterial genera (average GC content of 34%)	14
		Module A (7.3 kb), located at the 5′ end of the element, contains a restriction-modification system.
		Module B (39.5 kb) contains a complete prophage of Thermoanaerobacter sp. strain 513X (CP002210).
		Module C (4.5 kb) contains part of plasmid originating from E. faecalis (pEF418 [AF408195.1]) including spectinomycin adenyltransferase gene ant9-Ia.
		Module D (4.5 kb) is located entirely within module E, shows homology to a pathogenicity island of Campylobacter fetus, and harbors tet-44 and ant6-Ib genes.
		Module E (51 kb) is located at the 3′ end of the element and homologous to an entire conjugative transposon from Streptococcus pneumoniae, Tn1806.
Tn1549 (KU558763)	vanB2 operon (vanXB, vanB, vanHB, vanW, vanYB, vanSB, and vanRB): biosynthesis of modified peptidoglycan precursors (e.g., d-Ala-d-Lac or d-Ala-d-Ser) to which vancomycin shows reduced binding	42,375 bp in size and 38 ORFs with homology and synteny with Tn1549 (AF129329)g	22
		Recombination module comprising int (1,194 bp) and xis (201 bp) genes
		The central AMR domain comprises the vanB2 operon with syntenic vanXB, vanB, vanHB, vanW, vanYB, vanSB, and vanRB genes.
		However, the vanRB genes (encoding a cytoplasmic response regulator) are interrupted by loci from Bacillus megaterium (Bm3R1 [582 bp]) and Bacillus cereus (orf7 [1,032 bp]), respectively.
		The aberrant vanRB genes are likely responsible for the cryptic phenotype observed (22).
		Defining the left (L) and right (R) terminal ends of the element were 11-bp inverted repeats matching those found in Tn1549 and likely representing excision/integration sites.

^aTn, transposon; MLS_B, macrolide-lincosamide-streptogramin B; int, integrase; xis, excisionase. ORF, open reading frame.

^bPreviously termed CTnCD3a (41).

^cTn5398, the predominant ermB-containing element in C. difficile (40).

^dSize based on reported homology to Tn916 (14).

^eTn5397, the predominant tetM-containing element in C. difficile (40).

^fPosition in C. difficile M120 genome (418525 to 525236).

^gTn1549, conjugative transposon linked with the emergence and global dissemination of vancomycin-resistant enterococci (22).