pTAC2, -6, and -12 Are Components of the Transcriptionally Active Plastid Chromosome That Are Required for Plastid Gene Expression

Jeannette Pfalz ¹, Karsten Liere ², Andrea Kandlbinder ³, Karl-Josef Dietz ³, and Ralf Oelmüller ¹^*

¹ Institute of General Botany and Plant Physiology, Friedrich-Schiller-University, 07743 Jena, Germany
² Institute of Biology (Genetics), Humboldt University, D-10115 Berlin, Germany
³ Department of Plant Physiology and Biochemistry, University of Bielefeld, 33501 Bielefeld, Germany

^* To whom correspondence should be addressed. E-mail: b7oera{at}uni-jena.de.

Transcription in plastids is mediated by a plastid-encodedmultimeric (PEP) and a nuclear-encoded single-subunit RNA polymerase(NEP) and a still unknown number of nuclear-encoded factors.By combining gel filtration and affinity chromatography purificationsteps, we isolated transcriptionally active chromosomes fromArabidopsis thaliana and mustard (Sinapis alba) chloroplastsand identified 35 components by electrospray ionization iontrap tandem mass spectrometry. Eighteen components, called plastidtranscriptionally active chromosome proteins (pTACs), have notyet been described. T-DNA insertions in three correspondinggenes, ptac2, -6, and -12, are lethal without exogenous carbonsources. Expression patterns of the plastid-encoded genes inthe corresponding knockout lines resemble those of ${Delta}$ rpo mutants.For instance, expression of plastid genes with PEP promotersis downregulated, while expression of genes with NEP promotersis either not affected or upregulated in the mutants. All threecomponents might also be involved in posttranscriptional processes,such as RNA processing and/or mRNA stability. Thus, pTAC2, -6,and -12 are clearly involved in plastid gene expression.