Transcription from plastid gene promoters is stimulated in vitro when supercoded DNA templates are used instead of relaxed circular or linear templates. Furthermore, DNA conformation affected the relative ratio of transcription of two adjacent promoters (atpB, rbcL) in vitro studies.
Chloroplasts of higher plants contain gyrase and topisomerase I activity which could alter the superhelicity of plastid DNA in vivo. To date, however, no direct test of this possibility in higher plants has been reported. In Chlamydomonas, inhibitors of gyrase, such as novobiocin, change the relative transcription rates of several plastid genes.
