IS200 is not a member of the IS600 family of insertion sequences.

IS200 is an insertion sequence present in all Salmonella species except S. agona (1, 2). The element is absent from all other Enterobacteriaceae, with the exception of some strains of Shigella flexneri and Shigella sonnei (2). It had been hypothesized that the first mutation characterized as an IS200 insertion, hisD984: :IS200 (1, 3) might actually be a deletion mutant of the Shigella insertion element IS630 (4). However, sequencing of the ends of a second mutation caused by IS200 insertion later suggested that IS200 was a distinct insertion element (5). We recently obtained the complete nucleotide sequence of the insertion element IS200 inserted in the histidine operon of S. typhimuriwn (causing the mutation hisD984: :IS200). The element is 708 bp long; if this size corresponds to a wild-type element, IS200 can be considered the smallest insertion sequence known (6). A diagram of the hypothetical structure of IS200 is presented in Figure 1. Unlike many insertion elements, IS200 lacks tenninal inverted repeats, as previously reported (3, 5). The element contains three potential open reading frames (ORF 1, ORF 2 and ORF 3), whose coding capabilities are 69, 71 and 58 amino acids, respectively. ORF2 and ORF3 are overlapping; the existence of overlapping ORFs is not surprising, since it seems to be extremely common among bacterial insertion sequences (6). Other interesting features of IS200 are the following: (i) Three potential hairpin loops are located near the 'right' end of the element (Figure 1). One of the loops, centered on base 688, strongly reminds of a Rho-independent transcription terminator, as previously suggested (3). Two smaller hairpins are centered on bases 608 and 665. (ii) Two nearly perfect inverted repeats of 20 bp exist within the sequence. Both repeats are located near the 'right' end, extending between nucleotides 565-584 and 629-648 (Figure 1). The repeats differ by only 4 mismatches and their potential biological significance is unknown. A search in the EMBL data bank has confirmed that IS200 is not a deletion mutant of IS630, although two discrete regions of homology (8 and 10 bp long) appear to exist between IS200 and IS630. Likewise, two short regions of homology (7 and 8 bp long) are found between IS200 and the Shigella sonnei insertion sequence IS640. Two DNA stretches, 10 and 11 bp long, homologous to the S. sonnei element IS600 have also been found. Probes lacking homology with the Shigella elements IS600, IS630 and IS640 can now be obtained by cloning specific, internal fragments of IS200. These probes are expected to be useful to find out whether the hybndization found between IS200 probes and certain Shigella DNAs (2) was caused or not by crosshybridization between IS200 and Shigella insertion elements.