Different means, same end-heterochromatin formation by RNAi and RNAi-independent RNA processing factors in fission yeast.

The assembly of heterochromatin in eukaryotic genomes is critical for diverse chromosomal events including regulation of gene expression, silencing of repetitive DNA elements, proper segregation of chromosomes and maintenance of genomic integrity. Previous studies have shown that noncoding RNAs and the RNA interference (RNAi) machinery promote the assembly of heterochromatin that serves as a multipurpose platform for targeting effectors involved in various chromosomal processes. Recent work has revealed that RNAi-independent mechanisms, involving RNA processing activities that utilize both noncoding and coding RNAs, operate in the assembly of heterochromatin. These findings have established that, in addition to coding for proteins, mRNAs also function as signaling molecules that modify chromatin structure by targeting heterochromatin assembly factors.

RNAi-dependent formation of heterochromatin and its diverse functions.

Expression profiling of eukaryotic genomes has revealed widespread transcription outside the confines of protein-coding genes, leading to production of antisense and non-coding RNAs (ncRNAs). Studies in Schizosaccharomyces pombe and multicellular organisms suggest that transcription and ncRNAs provide a framework for the assembly of heterochromatin, which has been linked to various chromosomal processes. In addition to gene regulation, heterochromatin is crucial for centromere function, cell fate determination as well as transcriptional and posttranscriptional silencing of repetitive DNA elements. Recently, heterochromatin factors have been shown to suppress antisense RNAs at euchromatic loci. These findings define conserved pathways that probably have major impact on the epigenetic regulation of eukaryotic genomes.