Eukaryotic Chromosome

The presence of enzymes that can alter the structure of chromatin was suggested by yeast genetic studies that identified a number of genes, called SWI and SNF genes. These genes are required for multiple transcriptional activation events. A key breakthrough came when it was discovered that yeast cells could compensate for a deficiency in these SWI and SNF gene products by altering their chromatin structure. This led to the hypothesis that SWI and SNF genes are involved in the regulation of chromatin structure. It is now known that SWI and SNF proteins form a large, multisubunit complex, termed SWI/SNF, that can hydrolyze ATP and use the energy thus generated to alter chromatin structure.

Similar proteins that can hydrolyze ATP are present throughout the eukaryotic kingdom, and these form related multiprotein enzymes that also possess chromatin-remodeling properties. The mechanism by which these complexes alter the chromatin structure is unclear, but it is likely that the enzymes break or loosen the linkage between histone and DNA in a manner that increases the mobility and flexibility of the DNA wrapped around the histone core. It is important to keep in mind that these enzymes can be involved both in gene-activation events, by facilitating the binding of transcriptional activators, and in gene-repression events, perhaps by facilitating the binding of a transcriptional repressor or by directly promoting compaction of the chromatin structure.