Abstract
Constitutive heterochromatin, mainly formed at the gene-poor regions of pericentromeres, is believed to ensure acondensed and transcriptionally inert chromatin conformation. Pericentromeres consist of repetitive tandem satelliterepeats and are crucial chromosomal elements that are responsible for accuratechromosome segregation in mitosis. Therepeat sequences are not conserved and can greatly vary between different organisms, suggesting that pericentromericfunctions might be controlled epigenetically. In this review, we will discuss how constitutive heterochromatin is formedand maintained at pericentromeres in order to ensure their integrity. We will describe the biogenesis and the functionof main epigenetic pathways that are involved and how they areinterconnected. Interestingly, recent findings suggestthat alternative pathways couldsubstitute for well-established pathways when disrupted, suggesting that constitutiveheterochromatin harbors much more plasticity than previously assumed. In addition, despite of the heterochromaticnature of pericentromeres, there is increasing evidence for active and regulated transcription at these loci, in a multitudeof organisms and under various biological contexts. Thus, in the second part of this review, we will address this relativelynew aspect and discuss putative functions of pericentromeric expression.