OPPOSING ROLES OF THE HOLLIDAY JUNCTION PROCESSING SYSTEMS OF ESCHERICHIA-COLI IN RECOMBINATION-DEPENDENT ADAPTIVE MUTATION

Aspects of the molecular mechanism of ''adaptive'' mutation are emergi ng from one experimental system: reversion of an Escherichia coli lac frameshift mutation carried on a conjugative plasmid. Homologous recom bination is required and the mutations resemble polymerase errors. Rep orts 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 cap ture the replicated fragment into an intact replicon. Whereas conjugat ional recombination uses either of two systems of Holliday junction re solution, we find that the adaptive lac reversions are inhibited by on e resolution system and promoted by the other. Moreover, temporary abs ence of both resolution systems promotes mutation. These results imply that recombination intermediates themselves promote the mutations.