Translocation of DNA segments is a recombinational event seen in both eukaryotic and prokaryotic chromosomes, and it is thought to be involved in controlling gene expression and in the evolution of chromosomes. In bacteria, insertion (IS) and transposable (Tn) elements not only translocate their own DNA, but also promote the rearrangement of both bacterial chromosomes and the plasmic genomes carrying them. The insertion element IS1 is one such element which is 768 base pairs long. IS1 is involved in the generation of deletion mutations and in the fusion of two different plasmid genomes. It can also promote the translocation of DNA segments flanked by two copies of IS1 to give rise to transposable elements responsible for antibiotic resistance and enterotoxin production. We report here the distribution of the IS1 sequence in various bacterial DNAs, particularly in the family Enterobacteriaceae. Comparison of the results with the phylogenetic relationship of these bacteria suggests that IS1 was transferred from one bacterium to another after their divergence and in some bacteria the copy number of IS1 increased by translocation. The increase in the number of copies of IS1 in bacteria may increase the probability of the genetic rearrangement responsible for the generation of resistance and enterotoxin plasmids, the existence of which is a serious problem in medical microbiology.

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This paper describes an evolutionary analysis of ISI elements based on nucleotide sequence data from six iso-insertion elements of ISJ (isoISIs) which are present in chromosomes and plasmids of Escherichia coli and Shigella species as repeated sequences. The sequence comparison, which permitted construction of a phenogram, showed that the iso-ISIs can be divided into three groups. One group consists of four elements with 1 % nucleotide sequence divergence. A second and third group each consists of one element, with 10 % and 46 % divergence, respectively, to the first group. Despite their divergence, amino acid sequences in the two ISI encoded genes, insA and insB, and the terminal inverted repeat sequences, insL and insR, were found to be highly conserved. The evolutionary distance per site in the six sequences suggests that the ISI element has diverged to a greater degree than bacterial genes of known nucleotide sequences have. We postulate that the existence of a group of well conserved iso-ISIs and of highly diverged iso-ISIs may be due to the transposition ability of the ISJ element, generating repetitive sequences in both bacterial chromosomes and plasmids which can then independently diverge. We also discuss possible regulatory mechanisms of transposition mediated by ISI based on this analysis, including the influence of colon usage of insA and insB. The observed colon selection agrees well with those colons used by weakly expressed E. coli proteins.The insertion element ISI is one of a number of transposable DNA elements known in bacteria. An ISJ which is present in the resistance plasmid 8100 as a natural component, and thus termed ISIR, is composed of 768 by (12,34,36,37). Our genetic analysis has demonstrated that IS1R codes for two genes, named insA Abbreviations: bp, basepairs; kb, kilobasepairs; Kd, kilodaltons. 359