The Plastid Nucleoid Proteome in Developing Maize and Arabidopsis Leaves; pTAC17 is a target for Clp-mediated degradation

Plastids contain multiple copies of the plastid chromosome, folded together=  with proteins and RNA into nucleoids. The degree to which components of th= e plastid gene expression and protein biogenesis machineries are nucleoid-a= ssociated, and the factors involved in plastid DNA organization, repair and=  replication are poorly understood. Here we provide a conceptual framework = for nucleoid function, through mass spectrometry-based quantitative charact= erization of nucleoid from proplastids and chloroplasts in maize leaves. Th= is nucleoid-enriched proteome included proteins involved in DNA replication= , organization and repair, as well as transcription, mRNA processing, splic= ing and editing. Many protein of unknown function including PPR, TPR, DnaJ = and mTERF proteins were identified. 70S ribosome and ribosome assembly fact= ors were strongly overrepresented in nucleoid fractions, but translation fa= ctors and protein chaperones were not. Our analysis strongly suggests that = mRNA processing, splicing and editing, as well as ribosome assembly, take p= lace in association with the nucleoid; suggesting that these processes occu= r co-transcriptionally. The plastid developmental state shifted the predomi= nating function from RNA metabolism in undeveloped plastids to translation = and homeostasis in chloroplasts. Using a similar MS-based approach, we dete= rmined that the Arabidopsis nucleoid-enriched proteome was highly comparabl= e to maize nucleoids. I will further highlight pTAC17, a protein with unkno= wn function, loosely-associated with the nucleoid. We show that pTAC17 is a=  target for Clp protease mediated-degradation through interaction with ClpS= 1.