Bacterial insertion sequences.

Abstract

While DNA has a property of being fundamentally stable as invariable genetic information, studies on gene expression and gene organization have revealed that the genome is often subject to dynamic changes. Some of these changes are brought about by mobile genetic elements which have been found in prokaryotic and eukaryotic genomes so far studied. Insertion sequences (ISs) are bacterial mobile DNA elements which cause various kinds of genome rearrangements, such as deletions, inversions, duplications, and replicon fusions, by their ability to transpose. These were discovered during investigation of mutations that are highly polar in the galactose and lactose operons of Escherichia coli K−12 (Jordan etal. 1968; Malamy 1966, 1970; Shapiro 1969) and in the early genes of bacteriophage λ (Brachet et al. 1970). Many of these mutations were shown by electron microscope heteroduplex analysis to be insertions of distinct segments of DNA which are hence called insertion sequences (Fiandt et al. 1972; Hirsch et al. 1972; Malamy et al. 1972). It was later shown that the transcription of flanking genes can originate from promoters located within an IS or from hybrid promoters created by the insertion event or by the IS-mediated genome rearrangements. An important note here is that the finding of IS elements as mobile elements to new loci to turn genes either off or on would re-evaluate the controlling elements described in maize by McClintock (1956, 1965) (see Starlinger and Saedler 1976).