Although all the cells in an organism have the same genome, different tissues diverge in their gene expression profile and over time the same tissue will show different gene expression. This differences in gene expression patterns can be explained through epigenetics. Chromatin modifications can affect a gene expression without modifications in the DNA sequence and can be responsible for the transition from euchromatin, more accessible chromatin with less condensed, to heterochromatin, a highly condensed state and transcriptionally silenced1. It is known that heterochromatin has many different functions like in regulation of gene expression, chromosome segregation and genome stability2.
The formation of this compacted structure is achieved by histone modification (acetylation, methylation) and is spreading through some complexes able to recruit some other proteins and in this way, mark the chromatin for condensation. Usually for studies related with heterochromatin and the mechanisms behind heterochromatin assembly the fission yeast Schizosaccharomyces pombe is used as a model system due to its identity to some mammals’ proteins required for heterochromatin formation3 and because of its chromatin structure. The genome of the fission yeast has heterochromatin blocks in pericentromeric repeats, telomeres and in the mating-type region4 and heterochromatin formation in these regions involve some proteins that will lead to posttranslational histones modifications. One of the pathways involved in heterochromatin assembly and silencing is the RNA interference (RNAi) pathway and requires the intervention/recruitment of an RNAi complex called RITS (RNA-induced initiation of transcriptional gene silencing)4. Previously it was thought that heterochromatin was transcriptionally silenced, however new studies shown that this is not the case and there is still some transcription, even in low levels, in these loci5.
However, through the RITS complex and interactions between the transcripts and RNAi, the transcripts originated from heterochromatin regions are degraded and therefore the heterochromatin is fully silenced.