Figure 1. . Structural organization of integrons carried by IncFI plasmids. Numbers above each lane indicate plasmid reference numbers as defined (Table). Lane C shows the plasmidless E. oli K-12 strain CSH26 (11). A: agarose (0.8%) gel electrophoresis in 1x Tris-borate-EDTA buffer of plasmid DNA extracted from E. coli K-12 exconjugants. DNA was stained with ethidium bromide and visualized under UV light. B: Southern blot hybridization of plasmids shown in panel A with the intI1 probe. C: Southern blot hybridization with the intI1 probe of PvuII-BamHI double-digested DNA from E. coli K-12 exconjugants. The DNA was extracted as described (7). The intI1 probe was amplified with primers corresponding to nt 4680-4700 and nt 5252-5232 of the released Tn21 sequence (GenEMBL accession no. AF071413). Plasmid pACYC184::Tn21 (10) was used as the template. The probe was labeled with [-³²P]dCTP by using a random priming kit (GibcoBRL). Chromosomal DNA fluoresced because of weak trapping of partially degraded plasmids. D: schematic representation of In-t2 and In-t1 integrons. The thick bars above each structure represent the DNA fragments recognized by the intI1 probe. The 4.0-kb PvuII-BamHI fragment (In-t2) encompasses the intI1 gene segment downstream of the conserved PvuII site (0.6 kb), the oxa1 and aadA1 gene cassettes (2.2 kb), and the 3'-CS comprising the sul1 gene upstream of the BamHI site (1.2 kb) (7,8). The 1.2-kb PvuII-BamHI fragment (In-t1) encompasses the intI1 gene segment and part of the aadB gene cassette (7).