In vivo analysis of the plasmid pAM beta 1 resolution system.

The promiscuous plasmid pAM beta 1 from Gram-positive bacteria encodes a resolution system which differs from that of Tn3 in that (i) it requires a histone-like protein and an unusual resolvase-DNA interaction to promote recombination and (ii) it mediates in vivo DNA inversion in plasmid substrates. In this in vivo analysis, the pAM beta 1 resolution site is narrowed down to a 99 bp segment, the strand exchange is mapped within 10 bp and the serine residue at position 10 of the resolvase is shown to be essential for enzyme activity. In addition, data showing that the resolution system does not promote DNA inversion in the Bacillus subtilis chromosome are presented. Implications of this observation are discussed.