IS5 was originally isolated as an insertion into the lambda KH100 cI gene (Blattner et al., 1974), and was also shown (by hybridization) to be present in Citrobacter freundii (Nisen et al., 1979) and Shigella flexneri (Shoner and Kahn, 1981). Although the IS5 copy number varies from 10 to 23 among the Escherichia coli K-12 strains, there is an extensive variation among other E. coli strains (ECOR and Murray collections), many of them been devoted of IS5 or having only a single copy (Green et al., 1984). 23 IS5 copies have also been mapped in the E. coli W3110 chromosome, including their orientation (Umeda and Ohtsubo, 1990): most copies were scattered over the entire chromosome, only 4 elements been clustered. IS5 is thought to be polar in both orientations. An IS distribution study among 23 closely related O1:K1 pathogenic E. coli strains showed an average of 9 IS2 copies/genome, as compared to the 41, 17, 4, and 3 copies/genome for IS1, IS4, IS5 and IS30, respectively (Lawrence et al., 1989). IS5 shows a clear preference for 5'-CWAR-3' target sequences. The role of the putative second and third ORF (ins5B and ins5C) located on the other strand (Rak and von Reutern, 1984) remains to be confirmed. IS5 has also been shown to behave as a transcriptional enhancer of the otherwise cryptic E. coli bgl operon (Schnetz and Rak, 1992). Muramatsu et al. (1988) have shown that IS5 contains a sharply curved DNA structure (sequence-directed bent DNA) centered at about 60-70 bp from its right end, in an AT-rich region.
IS5 is also present in Neisseria gonorrhoeae (Accession number : U20421), the sequence is 100% DNA similar to the sequence of the database.