Recombination Between Homologies in Direct and Inverse Orientation in the Chromosome of Salmonella: Intervals Which Are Nonpermissive for Inversion Formation

Sequences placed in inverse order at particular chromosome sites (permissive) recombine to generate an inversion; the same sequences, placed at other sites (nonpermissive) interact recombinationally but do not form the expected inversion recombinants. We have investigated the events that occur between sequences at nonpermissive sites. Genetically marked lac operons in inverse order were placed at nonpermissive sites in a single chromosome and Lac+ recombinants were selected. No inversions were formed. The Lac+ recombinants recovered include double-recombinant types in which information appears to have undergone a nonreciprocal information exchange; one mutant copy is repaired with no alteration of the other copy. Recombination within the lac operon is stimulated more than 100-fold by the presence of extensive homology (antenna sequences) outside of the region for which recombination is selected. Sequences placed in direct order at the ends of the same noninvertible chromosome segment recombine to form all the expected recombinant types including those in which a reciprocal exchange has generated a duplication. All the detected recombinant types can be accounted for by recombination between sister chromosomes. These results are discussed in terms of two alternative models. One explanation of the failure to detect inversion of some intervals is that particular inversions are lethal, despite the fact that no essential sequences are disrupted. Another explanation is that chromosome topology prevents sequences at nonpermissive sites in a single chromosome from engaging in the direct interaction required for inversion formation, but allows the sister strand exchanges that can generate the recombinant observed.