INSERTIONS OF IS256-LIKE ELEMENT FLANKING THE CHROMOSOMAL BETA-LACTAMASE GENE OF ENTEROCOCCUS-FAECALIS CX19

We have previously identified an inverted repeat characteristic of sta phylococcal beta-lactamase transposons adjacent to the chromosomal bet a-lactamase genes of Enterococcus faecalis CH19 and its beta-lactamase -producing transconjugant CX19. Nucleotide sequence analysis of the CH 19 beta-lactamase structural gene (blaZ) reveals it to be identical to the blaZ gene from E.faecalis HH22 and to the blaZ gene from the stap hylococcal beta-lactamase transposon Tn552. We also report the presenc e of nucleotide sequence identical to a 317-bp region of the staphyloc occal insertion sequence IS256 upstream of the blaZ gene in both CH19 and CX19. The identical segment of IS256 is present downstream of the blaZ gene of CX19, suggesting a second insertion of the element (in th e inverted orientation) accompanying transfer to the recipient strain. Restriction analysis of the areas beyond the ClaI sites used to clone these regions suggests that full copies of the IS256-like element (de signated IS256E) are present in all positions but that these elements were not directly involved in the transfer of the beta-lactamase gene to the recipient strain. We have also identified a region downstream o f the second IS256E insertion site which exhibits substantial homology to ISSIW, an iso-ISSI insertion sequence originally identified in Lac tococcus lactis subsp. cremoris. These data suggest that the two enter ococcal blaZ genes sequenced to date evolved from a common ancestor an d may at one time have been incorporated into a transposon similar to Tn552. They also suggest that IS256-like elements are mobile in E. fae calis and capable of inserting in a manner consistent with the formati on of novel composite transposons. Finally, they provide the first con firmation of the presence of an ISSI-like element in enterococci, rais ing the possibility that these elements play a role in the exchange of chromosomal antimicrobial resistance determinants.