PLASMID-BORNE CADMIUM RESISTANCE GENES IN LISTERIA-MONOCYTOGENES ARE PRESENT ON TN5422, A NOVEL TRANSPOSON CLOSELY-RELATED TO TN917

The complete (6,449-bp) nucleotide sequence of the first-described nat ural transposon of Listeria monocytogenes, designated Tn5422, was dete rmined. Tn5422 is a transposon of the Tn3 family delineated by imperfe ct inverted repeats (IRs) of 40 bp. It contains two genes which confer cadmium resistance (M. Lebrun, A. Audurier, and P. Cossart, J. Bacter iol. 176:3040-3048, 1994) and two open reading frames that encode a tr ansposase (TnpA) and a resolvase (TnpR) of 971 and 184 amino acids, re spectively. The cadmium resistance genes and the transposition genes a re transcribed in opposite directions and are separated by a putative recombination site (res). The structural elements presumed to be invol ved in transposition of Tn5422 (IRs, transposase, resolvase, and res) are very similar to those of Tn917, suggesting a common origin. The tr ansposition genes were not induced by cadmium. Analysis of sequences s urrounding Tn5422 in nine different plasmids of L. monocytogenes indic ated that Tn5422 is a functional transposon, capable of intramolecular replicative transposition, generating deletions. This transposition p rocess is probably the reason for the size diversity of the L. monocyt ogenes plasmids. Restriction analysis and Southern hybridization revea led the presence of Tn5422 in all the plasmid-mediated cadmium-resista nt L. monocytogenes strains tested but not in strains encoding cadmium resistance on the chromosome.