On the role of IS1 in the formation of hybrids between the bacteriophage P1 and the R plasmid NR1

Summary The genomes of bacteriophage P1 derivatives carrying drug resistance genes derived from an R plasmid NR1 were analysed by restriction cleavage and be DNA-DNA hybridization. Two representatives of a class of oversized P1CmSmSu phages were identified as P1 carrying the entire r-determinant of NR1 together with its two flanking, directly repeated IS1. In one case the r-determinant insertion is carried at the site of the residential IS1 of P1, in the other case it is transposed into another region of the P1 genome. Models postulate that the first type resulted from reciprocal recombination within IS1 elements and that the formation of the second type of P1-R hybrid depended both on IS1 mediated transposition and reciprocal recombination. Plaque forming P1Cm or P1CmSm phages are explained as IS1 mediated deletion derivatives of P1CmSmSu, although an alternative model postulates that sometimes P1Cm phages might result from two consecutive transposition events of only one IS1 without involving reciprocal recombination. Secondary P1 derivatives carrying only one IS1 at the site of the original r-determinant or of Cm insertions into P1 must have been produced by reciprocal recombination between the two IS1 flanking the insertions. An implication from this study, that any genetic material carried adjacent to an IS1 element may undergo passive transposition, is discussed.