IS200: an old and still bacterial transposon

is200 is a mobile element found in a variety of eubacterial genera, such as salmonella, escherichia, shigella, vibrio, enterococcus, clostridium, helicobacter, and actinobacillus. in addition, is200-like elements are found in archaea. is200 elements are very small (707-711 bp) and contain a single gene. cladograms constructed with is200 dna sequences suggest that is200 has not spread among eubacteria by horizontal transfer; thus it may be an ancestral component of the bacterial genome. self-restraint may have favored this evolutionary endurance; in fact, unlike typical mobile elements, is200 transposes rarely. tight repression of transposase synthesis is achieved by a combination of mechanisms: inefficient transcription, protection from impinging transcription by a transcriptional terminator, and repression of translation by a stem-loop mrna structure. a consequence of is200 self-restraint is that the number and distribution of is200 elements remain fairly constant in natural populations of bacteria. this stability makes is200 a suitable molecular marker for epidemiological and ecological studies, especially when the number of is200 copies is high. in salmonella enterica, is200 fingerprinting is extensively used for strain discrimination.