Possible insertion sequences in a mosaic genome organization upstream of the exotoxin A gene in Pseudomonas aeruginosa.

Nucleotide sequence and Southern hybridization data revealed a mosaic genome organization in a region that extends several thousand base pairs upstream of the exotoxin A (toxA) gene in Pseudomonas aeruginosa. An interstrain comparison of DNA in this region showed a pattern of alternating segments of homologous and nonhomologous sequences. Two nonhomologous elements, approximately 1 kilobase pair upstream of the gene in strains PA103 and Ps388, were characterized in more detail. The sequence elements, denoted IS-PA-1 and IS-PA-2 for the different strains, are about 1,000 and 785 base pairs long, respectively, and have 5-base-pair direct repeats at their boundaries, consistent with their being DNA insertion sequences. The distribution of these elements in 34 different strains was determined. IS-PA-1 was found in a single copy upstream of toxA in half of the strains and was found in two copies in four of the strains. Some strains contained neither element, and one strain carried both. The genome of another strain, WR5, which lacks toxA, was shown to contain a 350-base-pair region that was highly homologous to DNA sequences located just upstream of toxA in other strains. The WR5 genome lacked several kilobase pairs of DNA that was found both upstream and downstream of this homologous region in the other strains.