Two amino acid residues of transposase contributing to differential transposability of IS1 elements in Escherichia coli

Escherichia coli W3110 contains four types of ISI elements in the chromosome. Using an insertion element entrapping system, we collected 116 ISI plasmid insertion mutants, which resulted from a minimum of 26 independent ISI insertion events. All of them had insertions of ISI of the IS1A (IS1E and IS1G) type, Inspection of the transposase sequences of the four ISI types and the ISI of the resistance plasmid R100 showed that two amino acid residues, His-193 and Leu-217 of transposase, might contribute to differential transposability of ISI elements in W3110. The two amino acid residues of the transposase in IS1A (IS1E and IS1G) were altered separately by site-directed mutagenesis, and each mutant was found to mediate transposition at a frequency about 30-fold lower than that of IS1A (IS1E and IS1G), Thus, the assumption that His-193 and Leu-217 of transposase contribute to differential transposability of ISI elements in W3110 was confirmed.