Opposing roles of the Holliday junction processing systems of Escherichia coli in recombination-dependent adaptive mutation

Aspects of the molecular mechanisms of 'adaptive' mutation are emerging from one experimental system: reversion of an Escherichia coli lac frameshift mutation carried on a conjugative plasmid. Homologous recombination is required and the mutations resemble polymerase errors. Reports implicating a role for conjugal transfer proteins suggested that the mutation mechanism is ordinary replication error occurring during transfer synthesis, followed by conjugation-like recombination, to capture the replicated fragment into an intact replicon. Whereas conjugational recombination uses either of two systems of Holliday junction resolution, we find that the adaptive lac reversions are inhibited by one resolution system and promoted by the other. Moreover, temporary absence of both resolution systems promotes mutation. These results imply that recombination intermediates themselves promote the mutations.