High-resolution map of plastid-encoded RNA polymerase binding patterns demonstrates a major role of transcription in chloroplast gene expression

Abstract

Plastids contain their own genomes, which are transcribed by two types of RNA polymerases. One of those enzymes is a bacterial-type, multi-subunit polymerase encoded by the plastid genome. The plastid-encoded RNA polymerase (PEP) is required for efficient expression of genes encoding proteins involved in photosynthesis. Despite the importance of PEP, its DNA binding locations have not been studied on the genome-wide scale at high resolution. We established a highly specific approach to detect the genome-wide pattern of PEP binding to chloroplast DNA using plastid chromatin immunoprecipitation-sequencing (ptChIP-seq). We found that in mature Arabidopsis thaliana chloroplasts, PEP has a complex DNA binding pattern with preferential association at genes encoding rRNA, tRNA, and a subset of photosynthetic proteins. Sigma factors SIG2 and SIG6 strongly impact PEP binding to a subset of tRNA genes and have more moderate effects on PEP binding throughout the rest of the genome. PEP binding is commonly enriched on gene promoters, around transcription start sites. Finally, the levels of PEP binding to DNA are correlated with levels of RNA accumulation, which demonstrates the impact of PEP on chloroplast gene expression. Presented data are available through a publicly available Plastid Genome Visualization Tool (Plavisto) at https://plavisto.mcdb.lsa.umich.edu/.

Keywords: RNA polymerase; plastid; sigma factor; transcription.