Figure 1.

A diagram showing the organization of the integron in Tn21 (X12870), carrying the aadA1 cassette encoding streptomycin/spectinomycin resistance and the following qacEΔ1–sul1 combination downstream of attC (15,33). The integron integrase is encoded in the 5′-conserved sequence, to the left. Genes are shown in bold face horizontal arrows and recombination sites with an open arrowhead. Vertical dashed lines mark cassette borders. The lower panel shows the sequence of the attC site. The site is almost a perfect palindrome organized into two subsites of dyad symmetry (boxes R″ and L″ and boxes L′ and R′). The bold face vertical arrow denotes the primary crossing-over point and the thin vertical arrows secondary sites (27). The ‘t’ stands for top strand and ‘b’ for bottom strand. The nucleotides are numbered from the 3′ cassette border (vertical dashed line) in the 5′–3′ direction according to the bottom strand. Dashed horizontal arrows show the symmetry within each subsite and the thin horizontal arrows the palindromic appearance of the site that is disturbed by the five local interruptions (enlarged bases in bold face).