REPAIR OF HETERODUPLEX DNA-MOLECULES WITH MULTIBASE LOOPS IN ESCHERICHIA-COLI

The fate of heteroduplex molecules containing 5-, 7-, 9-, 192-, 410-, and 514-base loops after transformation of wild-type and various mutan t strains of Escherichia coli has been examined. No evidence for repai r was obtained for the wild type or for strains with mutations in the following genes: mutS, recA, recBC sbcBC, recD, recF, recJ, recN, recO , recR, recBC sbcBC recF uvrA, recG ruvC, ruvB, lexA3, lexA51, uvrA, n fo xth nth, polA(Ts), or pcnB. These results rule out the involvement of the SOS system and most known recombination and repair pathways. Re pair of heteroduplex molecules containing 410- and 514-base loops was observed when a 1-base deletion-insertion mismatch was present nearby. The repair of both the mismatch and the loops was directed by the sta te of dam methylation of the DNA chains and was dependent on the produ ct of the mutS gene. A high efficiency of repair (95%) was found even when the mismatch and the loops were 1,448 nucleotides apart. We concl ude that multibase loops in DNA can be removed only as a consequence o f corepair by dam-directed mismatch repair.