Genetic organization of the downstream region of the mecA element in methicillin-resistant Staphylococcus aureus isolates carrying different polymorphisms of this region

Abstract

We describe here the genetic organization of the mec element downstream of the mecA gene in 34 different methicillin-resistant Staphylococcus aureus (MRSA) clinical isolates carrying 13 of the most frequent polymorphisms of mecA and representing the major epidemic clones of MRSA. All polymorphisms carried three common genetic elements: the hypervariable region, a copy of IS431, and a unique 2-kb sequence (downstream constant segment, or dcs) for which no homologous sequences are found in data banks. Polymorphisms of the downstream region were shown to be caused by the presence of linearized plasmids flanked by insertion sequences (pUB110, pT181, and pI258) and the autonomous insertion sequence IS256.

FIG. 1

Molecular organization of the ClaI::mecA downstream polymorphisms. ClaI-mecA types are displayed on the left; between parentheses are the approximate sizes of the ClaI::mecA downstream vicinities in kilobases. ClaI digestion sites are indicated with vertical arrows. Coding sequences for each plasmid are indicated as described in the original references. ΔHVR, deleted HVR; dcs, downstream constant segment; Ins117, the 117-bp sequence flanked by two 15-bp direct repeats; orfX, insertion site of the mec element into the S. aureus chromosome as defined by Ito et al. (15). In pattern I organization the digestion sites used for cloning are indicated, as are the locations of primers used in the screening of the other vicinities (see Table 3 for primer sequences).

FIG. 2

Molecular organization of the ClaI::mecA upstream polymorphisms. ClaI-mecA types are displayed on the left; between parentheses are the approximate sizes of the ClaI::mecA upstream vicinities in kilobases. ClaI digestion sites are indicated with vertical arrows. Horizontal arrows represent coding sequences and their directions of transcription. The upstream ClaI::mecA fragments were studied by IPCR and sequencing of the amplified fragments.