An evolutionary analysis of iso-is1 elements from Escherichia coli and Shigella strains.

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Abstract
This paper describes an evolutionary analysis of IS1 elements based on nucleotide sequence data from 6 iso-insertion elements of IS1 (isoIS1s) which are present in chromosomes and plasmids of E. coli and Shigella spp. as repeated sequences. The sequence comparison, which permitted construction of a phenogram, showed that the iso-IS1s can be divided into 3 groups. One group consists of 4 elements with 1% nucleotide sequence divergence. A second and third group each consists of 1 element, with 10 and 46% divergence, respectively, to the first group. Despite their divergence, amino acid sequences in the 2 IS1 encoded genes, insL and insR, were highly conserved. The evolutionary distance/site in the 6 sequences suggests that the IS1 element has diverged to a greater degree than bacterial genes of known nucleotide sequences have. The existence of a group of well conserved iso-IS1s and of highly diverged iso-IS1s may be due to the transposition ability of the IS1 element, generating repetitive sequences in chromosomes and plasmids which can then independently diverge. Possible regulatory mechanisms of transposition mediated by IS1 based on this analysis, including the influence of codon usage of insA and insB, are discussed. The observed codon selection agrees well with those codons used by weakly expressed E. coli proteins.