Subnuclear positioning and interchromosomal clustering of the GAL1-10 locus are controlled by separable, interdependent mechanisms.

On activation, the GAL genes in yeast are targeted to the nuclear periphery through interaction with the nuclear pore complex. Here we identify two cis-acting "DNA zip codes" from the GAL1-10 promoter that are necessary and sufficient to induce repositioning to the nuclear periphery. One of these zip codes, GRS4, is also necessary and sufficient to promote clustering of GAL1-10 alleles. GRS4, and to a lesser extent GRS5, contribute to stronger expression of GAL1 and GAL10 by increasing the fraction of cells that respond to the inducer. The molecular mechanism controlling targeting to the NPC is distinct from the molecular mechanism controlling interchromosomal clustering. Targeting to the nuclear periphery and interaction with the nuclear pore complex are prerequisites for gene clustering. However, once formed, clustering can be maintained in the nucleoplasm, requires distinct nuclear pore proteins, and is regulated differently through the cell cycle. In addition, whereas targeting of genes to the NPC is independent of transcription, interchromosomal clustering requires transcription. These results argue that zip code-dependent gene positioning at the nuclear periphery and interchromosomal clustering represent interdependent phenomena with distinct molecular mechanisms.