LACK OF CHROMOSOME TERRITORIALITY IN YEAST - PROMISCUOUS REJOINING OFBROKEN CHROMOSOME ENDS

Various studies suggest that eukaryotic chromosomes may occupy distinc t territories within the nucleus and that chromosomes are tethered to a nuclear matrix. These constraints might limit interchromosomal inter actions. We have used a molecular genetic test to investigate whether the chromosomes of Saccharomyces cerevisiae exhibit such territorialit y. A chromosomal double-strand break (DSB) can be efficiently repaired by recombination between flanking homologous repeated sequences. We h ave constructed a strain in which DSBs are delivered simultaneously to both chromosome III and chromosome V by induction of the HO endonucle ase. The arrangement of partially duplicated HIS4 and URA3 sequences a round each HO recognition site allows the repair of the two DSBs in tw o alternative ways: (i) the creation of two intrachromosomal deletions or (ii) the formation of a pair of reciprocal translocations. We show that reciprocal translocations are formed approximately as often as t he pair of intrachromosomal deletions. Similar results were obtained w hen one of the target regions was moved from chromosome V to any of th ree different locations on chromosome XI. These results argue that the broken ends of mitotic chromosomes are free to search the entire geno me for appropriate partners; thus, mitotic chromosomes are not functio nally confined to isolated domains of the nucleus, at least when chrom osomes are broken.