RNA polymerase IV
RNA polymerase IV is an enzyme which synthesizes small interfering RNA (siRNA) in plants. Polymerase IV is specific to plants genomes and is required for the synthesis of over 90% of 24-nt heterochromatic siRNA. 
RNA polymerase silences the transposons and repetitive DNA in the siRNA pathway. The siRNA plays a major role in defending the genome against the invading viruses and transposable elements by RNA directed DNA methylation. 
Polymerase IV and ROS1 demethylase unlocks and recondenses the 5S rDNA chromatin, which is present in seed and used for the development of adult features in plants.  Polymerase IV is involved in setting the methylation patterns in the 5S genes during plant maturation.
In arabidopsis, polymerase IV is known to interact with an RNA-dependent RNA polymerase, RDR2 and the collective activity of Pol IV-RDR2 is thought to give rise to the short dsRNAs which can be diced in to 24 nt siRNAs by DCL3. 
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