Characteristics of IS401, a New Member of the IS3 Family Implicated in Plasmid Rearrangements in Pseudomonas cepacia

We have determined the nucleotide sequence of IS401, an insertion sequence implicated in rearrangements of a 170-kb cryptic plasmid from Pseudomonas cepacia . Our analysis focused on a 4066-bp plasmid fragment containing adjacent copies of IS401 and of IS408, an element reported previously to activate gene expression in P. cepacia. One objective was to determine if an apparent increase in the copy number of IS401 in strains carrying adjacent plasmid copies of these two elements might be due to readthrough transcription of an IS401 transposase gene from an outwardly directed promoter within IS408. This possibility was ruled out by nucleotide sequence analysis of the 4066-bp plasmid fragment, which indicated that the major open reading frames of IS401 were oriented in the direction of IS408. IS401 was 1316 bp in length and had 26-bp terminal inverted repeats flanked by 3-bp direct duplications of adjacent DNA. It was closely related to the IS3 family elements IS51 from P. savastanoi and IS3411 from Escherichia coli. Pertinent features of IS408 are also discussed.