Fig. 4.

Possible templates for mutagenic DSB repair by homologous recombination during stress. a An I-SceI-induced DNA DSB that occurs within a duplicated region (blue arrows) can be repaired by homologous recombination using the intact duplicated segment as homology donor. Because duplications are rapidly lost by homologous recombination, the two copies could subsequently be reduced to one, either mutant or not (not shown). b I-SceIinduced DSBs could also be repaired by homologous recombination using a sister DNA molecule. This route may be more prevalent in conjugative plasmids than chromosomes because of their somewhat higher copy number. c Error-prone repair of a collapsed replication fork during stress differs from I- Sce I cuts in having one DSE, whereas I- Sce I creates two. Also, although in principle either a sister (shown) or duplication (not shown) could be used for repair, the sister is always present. Single lines represent double-stranded DNA, dashed lines newly synthesized DNA, and red Xs DNA polymerase errors that become mutations. For figure in color, please refer to the online version of the article.