Plastid In Vitro Transcription

Plastids, the characteristic organelles of plant cells, contain their own DNA and a complete apparatus for transcription and translation (1). Although this is principally similar to what is known for mitochondria, the details of the plastid gene expression system appear to be more complex, both with regard to the number of genes and mechanisms involved. Moreover, unlike mitochondria, plastids exist in various tissue-specific and developmental stage-specific forms, each differing in its state of gene expression. The best-known plastid types are the chloroplasts found in green (lightgrown) tissue and the etioplasts that develop in higher plant seedlings when these are grown in the dark (2). In contrast to the mitochondrial RNA polymerase enzyme, highly purified chloroplast enzyme preparations consist of a multisubunit complex similar to those in bacteria and also in the nuclei of eukaryotic cells, but with considerable species-specific variations. It is still an open question whether multiple entirely distinct RNA polymerases exist within the chloroplast and other plastid forms. Alternatively, one common core enzyme could be surrounded by different specificity factors, which would account for multiple, but closely related, holoenzymes.