RNA-induced Transcription Silencing

The process of RNAi involves the RNA-induced inhibition of translation. Histone modification and resulting heterochromatin formational can also down-regulate genes pre-transcriptionally. This pre-transcriptional silencing is referred to as RNA-induced transcriptional silencing (RITS), and it is carried out by a set of proteins called the RITS complex. (Some of the components are the same as in translational RNAi so you should review that.) RITS is not very well understood currently but seems to involve the binding of siRNA (generated by Dicer) to transcripts expressed from a chromosome region. When siRNA binds to the transcript, RITS (which includes the Argonaute protein and a chromodomain protein called Chp1) is recruited and binds histones in the region of the chromosome from which the transcript was expressed. This leads, in some way, to the local formation of heterochromatin which results in gene silencing - and, thus, silencing of the gene from which the transcript was expressed. In some organisms, this process appears to be involved in heterochromatin formation around centromeres and it also appears to play a role in X chromosome inactivation.