MOLECULAR ANALYSIS OF A MOBILIZABLE THETA-MODE TRIMETHOPRIM RESISTANCE PLASMID FROM COAGULASE-NEGATIVE STAPHYLOCOCCI

The Staphylococcus epidermidis plasmid pSK639 is the prototype of a ne wly described family of small plasmids identified in clinical staphylo coccal isolates. pSK639 is 8 kb in length and possesses a composite st ructure consisting of an IS257-flanked segment mediating trimethoprim resistance (Tp(r)), and regions responsible for replication and mobili zation of the plasmid. Comparative sequence analysis suggests that a p SK639-like plasmid may represent a progenitor of previously identified staphylococcal Tp(r) determinants related to the transposon-like stru cture, Tn4003. In contrast to the small staphylococcal plasmids charac terized to date that all utilize a rolling circle mode of replication, the replication region of pSK639 was found to contain features typica l of an iteron-controlled theta-mode replicon. pSK639 is the first sma ll plasmid of this type to be identified in the staphylococci. (C) 199 7 Academic Press.