BIASED SHORT TRACT REPAIR OF PALINDROMIC LOOP MISMATCHES IN MAMMALIAN-CELLS

Mismatch repair of palindromic loops in the presence or absence of sin gle-base mismatches was investigated in wild-type and mismatch-binding defective mutant Chinese hamster ovary cells. Recombination intermedi ates with a maximum heteroduplex DNA (hDNA) region of 697 bp contained a centrally located, phenotypically silent 12-base palindromic loop m ismatch, and/or five single-base mismatches. In wild-type cells, both loops and single-base mismatches were efficiently repaired (80-100%). When no other mismatches were present in hDNA, loops were retained wit h a 1.6-1.9:1 bias. However, this bias was eliminated when single-base mismatches were present, perhaps because single-base mismatches signa l nick-directed repair. In the multiple marker crosses, most repair tr acts were long and continuous, with preferential loss of markers in ci s to proximal nicks, consistent with nicks directing most repair in th is situation. However, similar to 25% of repair tracts were discontinu ous as a result of loop-specific repair, or from segregation or short tract repair of single-base mismatches. In mutant cells, single-base m ismatches were repaired less frequently, but the loop was still repair ed efficiently and with bias toward loop retention, indicating that th e defect in these cells does not affect loop-specific repair. Repair t racts in products from mutant cells showed a nide variety of mosaic pa tterns reflecting short regions of repair and segregation consistent w ith reduced nick-directed repair. In mutant cells, single-base mismatc hes were repaired more efficiently in the presence of the loop than in its absence, a likely consequence of corepair initiated at the loop.