RecO and the generation of merodiploids by transformation.

(A) Diagrammatic representation of the formation of merodiploids by transformation. This model involves ‘alternative pairing’ of a repeat sequence (R₁) within the transforming ssDNA, i.e. pairing not with its chromosomal counterpart but with a similar repeat (R₂) on one arm of a partially replicated recipient chromosome, coupled with ‘normal pairing’ of the non-repeat flanking ssDNA (A) on the other chromosome arm (next to the true chromosomal counterpart of R₁). This bridges the two chromosome arms, creating a chromosome dimer. It is of note that this dimer differs from 'simple' chromosome dimers made of two directly repeated monomers. Resolution of this 'rearranged' chromosome dimer generates one merodiploid chromosome with the region between repeats duplicated and another chromosome lacking this region [27] (panel B). (B) Chromosome dimer resolution can be mediated by XerS or by homologous recombination, where RecA could be loaded by RecO. The duplicated region is shown in green. Δ, deletion; †, abortive chromosome. (C) Stimulation of merodiploid formation by transformation in wildtype cells (R246). (D) Stimulation of merodiploid formation by transformation in recO mutant cells (R3170).