THE PROCESSING OF DNA ENDS AT DOUBLE-STRAND BREAKS DURING HOMOLOGOUS RECOMBINATION - DIFFERENT ROLES FOR THE 2 ENDS

We have investigated the role of DNA ends during gap repair by homolog ous recombination. Mouse cells were transfected with a gapped plasmid carrying distinctive ends: on one side mouse LINE-1 repetitive sequenc es (L1Md-A2), and on the other rat LINE-1 sequences (L1Rn-3). The gap could be repaired by homologous recombination with endogenous mouse ge nomic LINE-1 elements, which are on average 95% and 85% homologous to L1Md-A2 and L1Rn-3 ends, respectively. Both L1Md-A2 and L1Rn-3 ends we re found to initiate gap repair with equal efficiency. However, there were two types of gap repair products - precise and imprecise - the oc currence of which appears to depend on which end had been used for ini tiation and thus which end was left available for subsequent steps in recombination. These results, together with sequence analysis of recom binants obtained with plasmids having either mouse or rat LINE-1 seque nces flanking the gap, strongly suggest that the two DNA ends played d ifferent roles in recombinational gap repair. One end was used to init iate the gap repair process, while the other end was involved at later steps, in the resolution of the recombination event.