Heterochromatin formation and transcription in relation to trans-inactivation of genes and their spatial organization in the nucleus.

The role of transcription in heterochromatin formation in the nuclei of eukaryotes, originally shown for the pericentromeric heterochromatin assembly in fission yeasts, has now become an accepted paradigm extended to multicellular eukaryotes. It has been shown that small RNAs involved in the RNA interference system in its broadest sense can play an important role in this multi-step process - they are recognized by complementary interactions with the newly formed nuclear transcripts and recruit protein complexes to the local genomic sites for heterochromatinization. The role of transcription as a trigger of this process at the sites of genomic repeats will be considered in this review using various examples of heterochromatin formation, with an emphasis on discussion of its role in trans-chromosomal interactions causing gene inactivation.