pTAC3 and pTAC14 are required for binding of plastid-encoded RNA polymerase to DNA

Plastid-encoded RNA polymerase (PEP) is a bacterial-type multisubunit RNA polymerase responsible for the majority of transcription in chloroplasts. PEP consists of four core subunits, which are orthologs of their cyanobacterial counterparts. In Arabidopsis thaliana , PEP associates with 12 PEP-associated proteins (PAPs), which serve as peripheral subunits of the RNA polymerase. The exact contributions of PAPs to PEP function are still poorly understood. We use ptChIP-seq to show that PAP1/pTAC3, a peripheral subunit of PEP, binds to the same genomic loci as RpoB, a core subunit of PEP. The pap1 / ptac3 mutant shows a complete loss of RpoB binding to DNA throughout the genome, indicating that PAP1/pTAC3 is necessary for RpoB binding to DNA. A similar loss of RpoB binding to DNA is observed in the pap7 / ptac14 mutant, which is defective in another peripheral PEP subunit. We propose that the peripheral subunits of PEP are required for the recruitment of core PEP subunits to DNA.