Intercalary heterochromatin and genetic silencing.

We focus here on the intercalary heterochromatin (IH) of Drosophila melanogaster and, in particular, its molecular properties. In the polytene chromosomes of Drosophila, IH is represented by a reproducible set of dense bands scattered along the euchromatic arms. IH contains mainly unique DNA sequences, and shares certain features with other heterochromatin types such as pericentric, telomeric, and PEV-induced heterochromatin, the inactive mammalian X-chromosome and the heterochromatized male chromosome set in coccids. These features are transcriptional silencing, chromatin compactness, late DNA replication, underrreplication or elimination in somatic cells, and formation of the heterochromatin state in early embryogenesis. Post-translational modification of histones and the specific nonhistone protein complexes are shown to participate in the establishment and maintenance of silencing for all heterochromatin types. Many IH regions contain binding sites for HP1 and/or Pc-G proteins and all the regions are sites of heterochromatin-associated SuUR protein. Some IH regions are known to contain homeotic genes. Summarizing these data, we suggest that IH regions comprise stable inactivated genes, whose silencing is developmentally programmed. Copyright 2003 Wiley Periodicals, Inc.