A 2568-base pair (bp) insertion sequence (IS53) was cloned from the iaa-containing plasmid pIAA2 of Pseudomonas syringae subsp. savastanoi. IS53 is located downstream from the iaa operon and is inserted into IS51, a previously described element on pIAA2. IS53 causes an 8-bp target duplication and has nearly perfect terminal inverted repeats of 27 bp. Several degenerate copies of the inverted repeats are located internal, and in direct orientation to the terminal inverted repeats. A putative transposase gene conforms to the transposase ORF/opposite strand overlapping ORF motif described in many gram-negative IS elements. The region 5' to the presumptive transposase gene contains sequences that are homologous to the consensus E. coli heat-shock (sigma 32) promoter.