Plastid transcription: a menage a trois?

Two extracellular transcriptional compartments, mitochondria and plastids. are active in plants, depending mainly on nucleus-encoded proteins for tran scription. Several nuclear genes encoding RNA polymerases have been cloned from a variety of organisms and their subcellular localization studied in d etail. Surprisingly. the transit peptide of a recently published phage-type RNA polymerase from Arabidopsis(RpoT2) possesses dual targeting properties to both mitochondria and chloroplasts, inferring involvement in the transc ription of two different genomes. Possible implications of such a complex s ystem are discussed in this article.