Introduction - Shedding new light on the dark corners of the nucleus

Is it possible to distill any conclusions from these papers concerning heterochromatin and the chromosome mechanisms of gene expression and chromosome transmission? Well, we can agree with Laird that there is more than one heterochromatin (Laird, 1992). At the very least the distinction between facultative and constitutive heterochromatin (Brown, 1966) is real. The multiplicity of heterochromatins most likely reflects the inclusive nature of its definition as cytologically condensed material in interphase. The manifold bases for structural condensation are now being attacked at the genetic and molecular levels; a common theme that emerges is that in many cases, tandem arrays of repeated DNA sequences can recruit protein complexes locally within chromosomes to give rise to constitutive heterochromatin. There appears, however, to be no specific consensus sequence required for organizing constitutive heterochromatin, and distinct protein complexes may result in the same cytological and genetic properties. Proteins so far identified among constitutive heterochromatin complexes often appear to be evolutionarily conserved, arguing that the conservation of cytological appearance may have a conserved molecular basis. In a meeting report, Pardue and Hennig (1990) asked of heterochromatin the question posed by any visitor to a yard sale, estate auction or flea market. Is it junk, merely the accretion of discarded DNA, or is it a collectors item, something of value to the cell, and to students of nuclear function? Some regions of heterochromatin may indeed be 'junk', although one should be cautious in making this speculation. Clearly though, heterochromatic regions also contain essential functions (protein-coding sequences, rDNA, HeTA and TART retroelements, telomeres, achiasmate meiotic pairing sites and centromeres).