Transcription of all bacterial genes is performed by one core RNA polymerase consisting of 4 subunits (two a, one ß, and one ß'; e.g. in E. coli), or 5 subunits in case of the cyanobacteria which have ß' split into two subunits, ß' and ß'' (Kaneko et al. 1996). The plastid chromosomes of algae and higher plants posses genes for core subunits of a cyanobacterial-type RNA polymerase, first reported for Marchantia, tobacco and spinach (Ohyama et al. 1986; Shinozaki et al. 1986; Sijben-Muller et al. 1986), which is commonly abbreviated as PEP (for plastid-encoded plastid RNA polymerase; Hajdukiewicz et al. 1997; see Section 1.2; Fig. 1). The existence of one or more nuclear-encoded plastid RNA polymerase(s) (NEP) was suggested by comparing the effects of inhibitors of translation on cytoplasmic and plastidial ribosomes, respectively (Ellis and Hartley 1971). Detection of RNA polymerase in plastids with impaired protein synthesis implies a nuclear location of the gene(s) encoding this activity. Ribosome-deficient plastids isolated from heat-bleached rye leaves were found to exhibit RNA polymerase activity (Bünger and Feierabend 1980), and the detection of mature rRNAs in plastids that lack
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