Conserved Structure of IS200 Elements in Salmonella

Sequence analysis of three IS200</it> elements (two from Salmonella typhimurium</it>, one from Salmonella abortusovis</it>) reveals a highly conserved structure, with a length of 707–708 bp and absence of terminal repeats. IS200</it> contains an open-reading-frame (ORF) which potentially encodes a peptide of 151 amino acids, with a putative ribosome-binding-site properly placed upstream of the ORF. A potential RNA stem-loop structure that might occlude the ribosome-binding-site of the ORF is also found. Another conserved trait is a potential RNA hairpin which resembles a Rho-independent transcription terminator, located near one end of IS200</it>. The junctions between IS200</it> and host DNA sequences are A+T-rich. Upon insertion, IS200</it> duplicates 1–2 bp of host DNA sequences. The observation that IS200</it> elements characterized as ‘hops’ are roughly identical to those residing in the Salmonella</it> genome suggests that IS200</it> transposition is unlikely to generate inactive copies. If such is the case and many or all IS200</it> elements are active, the extremely low frequency of IS200</it> transposition may reflect the normal behavior of the element.