IS 186 Insertion at a Hot Spot in the lon Promoter as a Basis for Lon Protease Deficiency of Escherichia coli B: Identification of a Consensus Target Sequence for IS 186 Transposition

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Abstract
The radiation sensitivity of Escherichia coli B was first described more than 50 years ago, and the genetic locus responsible for the trait was subsequently identified as lon (encoding Lon protease). We now show that both E. coli B and the first reported E. coli K-12 lon mutant, AB1899, carry IS 186 insertions in opposite orientations at a single site in the lon promoter region and that this site represents a natural hot spot for transposition of the insertion sequence (IS) element. Our analysis of deposited sequence data for a number of other IS 186 insertion sites permitted the deductions that (i) the consensus target site sequence for IS 186 transposition is 5′-(G) ≥4 (N) 3–6 (C) ≥4 -3′, (ii) the associated host sequence duplication varies within the range of 6 to 12 bp and encompasses the N (3–6) sequence, and (iii) in a majority of instances, at least one end of the duplication is at the G-N (or N-C) junction. IS 186 -related sequences were absent in closely related bacterium Salmonella enterica serovar Typhimurium, indicating that this IS element is a recent acquisition in the evolutionary history of E. coli.