chloroplast, plastid, transcription, transcriptionally active chromosome (TAC)
In chloroplasts, the genetic material is organized as nucleus-like structures called nucleoids. Plastid nucleoids are anchored to the plastid envelope or thylakoid membranes and contain packaging proteins but also nucleic acid-related enzymatic activities. As early as in 1969 it has been reported that chloroplast DNA dependent RNA polymerase activity is associated with the thylakoid membrane fraction, and is therefore part of the nucleoids. This biochemical fraction, termed the Transcriptionally Active Chromosome (TAC), contains the transcriptional apparatus in tight connection with the plastid DNA, which distinguishes it from the RNA polymerase in the soluble fraction (sRNAP). In contrast to the latter, the TAC-associated RNAP was found to only elongate transcripts that have been initiated already in vivo. Genetic evidence emanating in the nineties indicated that chloroplasts possess two enzymes encoded by different genomes: one encoded by the nuclear genome (nuclear-encoded RNAP=NEP) and one plastid encoded RNAP (PEP). Significant efforts were conducted to find out how the genetic and biochemical data could be reconciled with each other and to elucidate whether the RNAP in the TAC fraction differs molecularly from the soluble RNAP of chloroplasts. Taken together results of these efforts and recent proteomic data revealed that PEP is the principle RNAP of TAC. In this report, an account of the accomplishments concerning the activities, the composition and the functional relevance of the TAC is presented from a historical perspective.