A great deal of information can be gained from an examination of the evolution of the subunit composition of the SWI/SNF or BAF complexes (Fig. 1). A clear transition occurs with the appearance of multicellularity, which can be appreciated by the additional complexity in multicellular organisms. Four subunits are lost from the yeast complex, and three different ones are added in flies (and probably also worms, although the complex in worms has not been purified). These changes in SWI/SNF probably reflect newly emerging strategies of chromatin regulation that must support the fact that whereas most genes are expressed in yeast, most genes are repressed in multicellular organisms. Although Polycomb complexes appeared evolutionarily before the split of plants and animals, they were lost in Saccharomyces cerevisiae and Saccharomyces pombe (13). Genetic studies in flies indicate that the major function of the BAP complex is to oppose Polycomb, whereas in yeast, genetic studies clearly demonstrated that histones and presumably nucleosomes are the major targets for SWI/SNF action. Several other strategic changes to chromatin occur in multicellular organisms, including the appearance of histone
