IS200 is not a member of the 1S600 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 presente