Largest subunit (NRPD1) of Higher plant RNA polymerase IV, C-terminal domain
Higher plants have five multi-subunit nuclear RNA polymerases: RNAP I, RNAP II and RNAP III, which are essential for viability; plus the two isoforms of the non-essential polymerase RNAP IV (IVa and IVb), which specialize in small RNA-mediated gene silencing pathways. RNAP IVa and/or RNAP IVb might be involved in RNA-directed DNA methylation of endogenous repetitive elements, silencing of transgenes, regulation of flowering-time genes, inducible regulation of adjacent gene pairs, and spreading of mobile silencing signals. NRPD1a is the largest subunit of RNAP IVa, whereas NRPD1b is the largest subunit of RNAP IVb. The full subunit compositions of RNAP IVa and RNAP IVb are not known, nor are their templates or enzymatic products. However, it has been shown that RNAP IVa and, to a lesser extent, RNAP IVb are crucial for several RNA-mediated gene silencing phenomena.