IS2 insertion is a major cause of spontaneous mutagenesis of the bacteriophage P1: non‐random distribution of target sites.

Insertion mutations arising spontaneously in the P1 prophage and affecting vegetative phage reproduction were screened for the presence of insertion sequence 2 (IS2). Filter hybridization identified 28 out of 44 independent insertions as IS2. Their target specificity is not random. A region that amounts to < 2% of the phage genome had trapped 15 of the 28 IS2 elements. However, precise mapping of nine mutants in this hot spot segment revealed no preferred insertion site. Rather, the nine IS2 are distributed over the whole target segment and IS2 are found in both orientations. Sequence data indicate that at least two sequence variants of IS2 participated in mutagenesis of the phage genome. The detectable transposition of IS2 from the host chromosome to the prophage occurs with a frequency of 3 x 10(‐5) per cell per generation under the particular experimental conditions. It is concluded that IS2, a natural resident of Escherichia coli K12 strains, is an important agent for spontaneous mutagenesis and exerts this action non‐randomly along the genome.