FIG. 1.

(A) Simplified diagram of the type I integron in Tn21 to illustrate integron organization and location of the att sites. (B) Nucleotide sequence of the IntI1 primary sites (attI and attC) in plasmids pSU18R2 and pSU18R3. The DNA sequences show the GTTAG crossover site at the right (3′) end. This orientation defines the top and bottom nomenclature for the ssDNAs. Filled vertical arrows indicate the crossover sites, and open horizontal arrows point to the four putative pentanucleotide components (boxed and numbered) of each att site. The numbers below the bottom strand of the aadA1 attC site start at the beginning of the reverse M13 priming site of the pSU18 vector sequence and correspond to numbers in Fig. 6. The shaded area indicates the region protected by IntI1 from DNase I attack.