The Su(Hw) chromatin insulator protein alters double-strand break repair frequencies in the Drosophila germ line

P element-induced double-strand breaks (DSBs) on the X chromosome of Drosop hila, melanogaster were repaired up to four times more frequently when func tional Su(Hw) chromatin insulator protein was removed from all genomic bind ing sites. Simultaneous comparisons of interallelic gap repair frequencies at two target loci on the X chromosome confirmed that a Su(Hw) binding site nested within a template had no effect on DSB repair efficiency. The resul ts suggest that the genome-wide homology search of broken ends for homologo us template sequences is affected because it is the only step in the recomb inational repair process with an apparent genome-wide interaction. We propo se that the searching 3'-hydroxy ends gain a higher degree of freedom for t he search in a su(Hw) mutant background.