The plastid genome in higher plants encodes subunits of an Escherichia coli-like RNA polymerase which initiates transcription of plastid genes from sequences resembling E.coli sigma70-type promoters. By deleting the gene for the essential beta subunit of the tobacco E.coli-like RNA polymerase, we have established the existence of a second plastid transcription system which does not utilize E.coli-like promoters. In contrast to the E.coli-like RNA polymerase, the novel transcription machinery preferentially transcribes genetic system genes rather than photosynthetic genes. Although the mutant plants are photosynthetically defective, transcription by this polymerase is sufficient for plastid maintenance and plant development.