CHROMATIN COMPLEXES AS APERIODIC MICROCRYSTALLINE ARRAYS THAT REGULATE GENOME ORGANIZATION AND EXPRESSION

The current understanding of chromatin-mediated repression in Metazoa stems largely from work on two systems in Drosophila: heterochromotin- induced position-effect variegation and repression of the homeotic gen es by the Polycomb-group of genes. A common feature of these two syste ms is the cooperative assembly of multimeric complexes which con epige netically silence gene activity. Moreover, both older and more recent work has suggested that these complexes can themselves associate to gi ve rise to larger complexes: The specificity of the association is lik ely to be determined by complementarity of the structural components o f the complexes. Here, we aim to accommodate these, and other, feature s of chromatin-mediated repression in a single hypothesis, namely the crystallisation hypothesis. This hypothesis views the nucleus as being an environment that favours the formation of chromatin complexes whic h behave as aperiodic microcrystalline arrays constructed through the cooperative assembly of different types of lattice unit. The lattice u nits possess regions of structural complementarity that allow. interac tions between complexes. Aperiodicity confers specificity on the compl exes and is a key feature of the model which, we suggest, provides a g ene with a ''chromosomal address.'' The chromosomol address allows the side-by-side alignment of homologous chromosomal regions, a property that may be important in a variety of biologically relevant situations . Aperiodicity is also a feature of the hypothesis that is directly te stable. (C) 1998 Wiley-Liss, Inc.