Expression of Plastid Genes by the Two RNA Polymerases

Abstract

Plastid genes are transcribed by two distinct RNA polymerases: the plastid-encoded plastid RNA polymerase (PEP) and the nuclear-encoded plastid RNA polymerase (NEP). The genes of the multi-subunit PEP core are encoded by the plastid genome, and are homologous to the eubacterial (E. coli-like) DNA-dependent RNA polymerase α, β and β‘ subunits (1, 2). The σ-like factors required for promoter recog.nition (3, 4) are encoded in the nucleus (5–8). PEP promoters are similar to eubacterial σ70-type promoters: the core is comprised of two hexameric sequences corresponding to the eubacterial -35 (TTGACA) and -10 (TATAAT) promoter elements. The hexamers are spaced 17–19 nucleotides apart; transcription initiates 5–7 nucleotides downstream of the -10 box sequence (1, 2). The catalytic subunit of NEP is related to the mitochondrial and phage T3/T7 RNA polymerases (9, 10). Promoters for the NEP fall in two classes. Type I NEP promoters are contained in ~15 bp upstream (-14 to +1) of the transcription initiation site (+1). Type H NEP promoters are located downstream (-5 to +25), with no sequence conservation between the two (11). NEP recognizes two distinct promoters, therefore it is assumed that the catalytic core associates with additional factor(s) conferring promoter specificity (11). PEP is derived from the RNA polymerase of the ancestral eubacteria-like endosymbionts of plastids. It is assumed that the phage-type plastid RNA polymerase evolved by duplication of the nuclear gene encoding the mitochondrial enzyme, and re-targeting of the gene product to plastids (10).