FIG. 2.

XbaI restriction site polymorphism in O157 strains attributable to an insertion in the virulence plasmid. (a) Comparison of pO157 DNAs from strain EDL933 (933); isolates G5320, G5327, G5303, and G5323; and strain 86-24. The arrows indicate the directions of transcription of the designated genes. Identical regions are shown as solid, and the inserts that differ among the strains are shown as open. The insertions in isolates G5303 and G5323 are identical but differ from that in strain 86-24. The insertion in strain 86-24 contains an XbaI site. Fragment IK8 (shaded), amplified by primer pair IK8A and -B, maps to a 635-bp insertion in an intergenic region in pO157 DNA from strain 86-24. The insertions in isolates G5303 and G5323 occur at positions in the intergenic region identical to that in strain 86-24. (b) Original primers (shown in boldface) and additional primers used for further analysis of the polymorphisms between strains. The primers are in direct alignment with the regions in pO157 DNA from strain 86-24 used to design them. (c) Agarose gel electrophoresis pattern of amplicons derived using these primer pairs. The results confirm the polymorphisms between strains 86-24 and EDL933 diagrammed in panel a. M, molecular size marker (100-bp DNA ladder; New England Biolabs, Inc., Beverly, Mass.); +, presence of amplicon; −, absence of amplicon.