The phage-type PclpP-53 plastid promoter comprises sequences downstream of the transcription initiation site

The existence of a phage-type plastid transcription machinery (NEP), related to the mitochondrial RNA polymerase, has been recognized only recently. Here we report the cis sequences required for transcription initiation by the phage-type enzyme. The promoter chosen for the study, PclpP-53, is well expressed in tobacco leaves, unlike most NEP promoters. Promoter definition was carried out in vivo, in transplastomic tobacco plants expressing a uidA reporter gene from PclpP-53 promoter derivatives. We report here that sequences from -5 to +25 (relative to the transcription initiation site) are sufficient to support specific transcription initiation. Requirement of sequences downstream of the transcription initiation site contrasts with mitochondrial promoters, which have conserved sequences predominantly upstream. The promoter defined here is conserved in liverworts and conifers, indicating that the phage-type transcription machinery appeared in plastids early on during the evolution of land plants. The PclpP-53 promoter sequences are present in rice but do not function, suggesting that PclpP-53 recognition specificity is absent in some monocots.