FIG. 1.
Structural features of ISPst6 in integrated and circularized forms. (A) Schematic representation of a segment of an integron-assembled gene cassette array showing the inverted repeat and PIRL sequences for an inserted IS1111-attC element. Note that the IS inserts in reverse orientation and the displayed sequence are for the sense strand of the IS but written 3′→5′ (i.e., as shown in panel B). Cassette-associated genes are represented by block arrows, and attC sites are represented by triangles. Components of the first gene cassette are colored gray, and components of the second gene cassette are colored black. The integron (not shown) is located upstream of the orfA gene cassette. (B) Linear diagram of ISPst6 as it occurs in cassette arrays. The location and orientation of the putative promoters PIRL and Pout, which were tested for activity in pUS90 and pUS92 constructs, are also shown. The sequence of the junction formed between IRR and IRL upon excision of ISPst6 as a circle is shown below. The recombination junction between the left and right termini (slash), the subterminal nature of the IR sequences (boldface letters with underline arrows), and the putative −35 and −10 sequences of the junction promoter (overlined) are all indicated. The start of the major transcript mapped in the pUS84 construct lies within the sequence CCCTG at the end of IRL.