siRNA-mediated transcriptional gene silencing: the potential mechanism and a possible role in the histone code

Epigenetics is the study of meiotically and mitotically heritable changes in gene expression which are not coded for in the DNA. Three distinct mechanisms appear intricately related in initiating and sustaining epigenetic modifications: RNA-associated silencing, DNA methylation and histone modification. Recently, in human cells small-interfering RNAs (siRNAs) have been shown to mediate transcriptional gene silencing (TGS). The observation that siRNAs can function to suppress gene expression at the level of transcription has created a major paradigm shift in mammalian RNA interference. The putative mechanism(s) of siRNA-mediated TGS in both yeast and human cells will be discussed. Undoubtedly, the ramifications from this paradigm shift in which RNA has demonstrated a potent and specific capability to regulate the expression of the gene are immeasurable both therapeutically (i. e. directed control of gene expression) and biologically in understanding the evolution of the cell.