Fig. 1. RNA polymerases in plastids. The nuclear-encoded plastid RNA polymerase (NEP) is related to phage-type single-subunit enzymes and may need additional, yet unknown protein factors for promoter recognition. The plastid-encoded plastid RNA polymerase (PEP) is a multisubunit enzyme homologous to bacterial RNA polymerases and consists of the core a2, ß, ß', and ß" subunits and the nuclear-encoded a-like factor required for promoter recognition. In chloroplasts, PEP associates with additional factors, which are thought to be involved in regulation of PEP transcription activity, including the plastid transcription kinase (PTK). The transcription initiation sites (TIS) are indicated by arrows.
ribosomes was reported as proof for accurately functioning nuclear-encoded RNA polymerase and rRNA processing activities in plastids of the barley mutant al-bostrians (Siemenroth et al. 1981). These early data have later been confirmed by demonstrating the expression of several genes in ribosome-free plastids of barley mutants (Hess et al. 1993), of heat-bleached rye leaves (Falk et al. 1993; Hess et al. 1993), and of the iojap mutant of maize (Han et al. 1993). Further evidence for NEP activity came from the detection of RNA synthesis in nonphotosynthetic plastids of the parasitic plant Epifagus virginiana (Ems et al. 1995). E. virginiana has a relatively small plastid genome that lacks genes for proteins involved in photosynthesis and, important in this context, for the core subunits of PEP (Morden et al. 1992). Similar observations have been made more recently with other parasitic plants, where transcription has to rely solely on NEP activity as their plastomes lack PEP genes (Lusson et al. 1998; Krause et al. 2003; Berg et al. 2004). The invention of genetic manipulation of plastid genes allowed for the directed inactiva-tion of PEP genes. Plants with deleted PEP genes still were able to transcribe their plastid genes, i.e., provided additional evidence for the existence of NEP. Moreover, the albino phenotype of these plants indicated that NEP activity alone is not sufficient for the development of photosynthetically active chloroplasts (Allison et al. 1996; Hajdukiewicz et al. 1997; Krause et al. 2000; Legen et al. 2002).
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