Turning genes off by Ssn6-Tup1: a conserved system of transcriptional repression in eukaryotes

The Ssn6-Tup1 repressor forms one of the largest and most important gene-regulatory circuits in budding yeast. This circuit, which appears conserved in flies, worms and mammals, exemplifies how a 'global' repressor (i.e. a repressor that regulates many genes in the cell) can be highly selective in the genes it represses. It also explains how, given the appropriate signal, specific subsets of these genes can be derepressed. Ssn6-Tup1 seems especially robust, bringing about a high level of repression irrespective of its precise placement on DNA or of specific features of the DNA control regions of its target genes. This high degree of repression probably results from several distinct mechanisms acting together.