A conserved Pol II elongator SPT6L mediates Pol V transcription to regulate RNA-directed DNA methylation in Arabidopsis

In plants, the plant-specific RNA polymerase V (Pol V) transcripts non-coding RNA and provides a docking platform for the association of accessory proteins in the RNA-directed DNA methylation (RdDM) pathway. Various components were uncovered in the process of DNA methylation, but it is still not clear how the transcription of Pol V is regulated. Here, we found the conserved elongation factor, SPT6L, bound to thousands of intergenic regions in an RNA polymerase II (Pol II) independent manner. The intergenic enrichment of SPT6L, interestingly, co-occupied with the largest subunit of Pol V (NRPE1) and the mutation of SPT6L led to the reduction of DNA methylation but not Pol V enrichment. Furthermore, the association of SPT6L at Pol V loci was dependent on the Pol V associated factor, SPT5L, rather than the presence of Pol V, and the interaction between SPT6L and NRPE1 was compromised in spt5l. Finally, Pol V RIP-seq revealed that SPT6L is required to maintain the amount and length of Pol V transcripts. Our findings revealed the critical role of a Pol II conserved elongation factor in Pol V mediated DNA methylation and transcription, and shed light on the mutual regulation between Pol V and Pol II in plants. 10-day old seedlings were used for ChIP-seq, BS-seq, RNA-seq, and RIP-seq assays.