Unraveling the complexities of plastid transcription in plants

Two RNA polymerases, plastid-encoded PEP and nuclear-encoded NEP, serve different sets of genes in plastids but share the transcription of housekeeping genes. Thus far, three genes have been identified that encode NEP -RpoTp, RpoTmp and RpoTm. The gene products of RpoTp and RpoTm are targeted to plastids and mitochondria, respectively. However, the RpoTmp was regarded as a dual targeting polymerase, although recent evidence demonstrates that the enzyme targets exclusively to mitochondria. This experimental discrepancy has important implications and changes our present understanding of the plastid transcription machinery.