Noncoding RNAs and chromatin structure.

A number of examples of noncoding RNA-connected chromatin modifications in eukaryotes has been recently revealed. Four cases are under detailed consideration in the present review, namely Xist RNA-dependent X-chromosome inactivation in mammals, roX RNA-dependent hyperactivation of X-chromosome in the fruit fly (in both cases the goal is dosage compensation, equalization of transcription level from two X chromosomes in females and one in males), and two examples of RNAi-connected down-regulation of transcription--siRNA-dependent heterochromatin formation in fission yeast and RdDM (RNA-dependent DNA methylation) in plants (FWA gene regulation in Arabidopsis). Although overall quite different, each phenomenon demonstrates some common features of RNA-driven chromatin modification process, including the role of RNA in aiming of chromatin-modifying protein complexes to their targets and subsequent formation of self-maintaining specific chromatin conformation (DNA methylation, changes in histone code, and binding of self-assembling protein complexes).