DNA-protein complexes that are capable of RNA synthesis in vitro (transcriptionally active chromosomes) were isolated from both chloroplasts and etioplasts of mustard (Sinupis alba L.) seedlings. Analyses of the polypeptide pattern of these complexes indicate that they comprise a specific subset of plastid proteins, distinct from the overall pattern of either the soluble or membrane-bound plastid proteins. DNA-protein complexes from the two plastid types have polypeptides in common. However, at least several polypeptides are highly enriched in either the chloroplast or the etioplast DNA-protein complex.The EcoRI restriction endonuclease fragments of the DNA associated with the complexes from either plastid type are the same. They are identical with the fragments obtained from highly purified chloroplast DNA.The transcriptional activity of the chloroplast complex is more than ten times higher than the activity of the etioplast complex. However, the complexes from either plastid type are capable of transcribing DNA regions containing genes for both the plastid rRNAs and for plastid proteins. In vitro transcripts were found to hybridize not only to DNA regions for mature in vivo RNA but also to adjacent regions, indicating synthesis of precursor RNA sequences by the transcriptionally active chromosomes.A typical feature of the light-mediated transition from etioplasts to chloroplasts is the induction of a specific set of ptDNA transcripts [I]. For example, light (via the photoreceptor phytochrome [2]) induces high levels of the mRNA for the M,-32000 PSII protein [3, 41. To resolve the mechanism of this differential gene expression, the availability of suitable in vitro transcription systems seems to be a prerequisite.One approach includes the development of homologous plastid transcription systems consisting either of purified RNA polymerase supplemented by an 'S-factor' [S, 61 or of crude plastid extracts [7-91 which are able to transcribe exogenous DNA templates. By using this functional assay, it has been possible to locate sequence elements within the 5'-flanking region of the PSII gene of mustard that are required for accurate transcription of this gene [9].

Mustard (Sinapis alba L.) seedlings were grown in the presence of herbicides (Difunon, Norflurazon) which inhibit carotenoid synthesis without affecting development, in darkness or in continuous far-red light. In strong white light (12,000 lx) the cotyledons of the herbicide-treated seedlings did not contain normal chloroplasts, but only small chlorophyll-free rudiments whose internal structure had almost disappeared. The plastid marker enzyme NADP-dependent glyceraldehyde-3-phosphate dehydrogenase was almost lacking. Plastid ribosomes and ribosomal RNAs were no longer detectable nor could synthesis of mature plastidal ribosomal RNAs be detected. Cytosolic ribosomes and rRNAs were not affected. Plastid DNA was apparently still intact as shown by restriction analysis. The appearance of marker enzymes of glyoxisomes, mitochondria and cytosol was not impaired while the level of marker enzymes of peroxisomes was drastically lowered. Accumulation of anthocyanin in mustard cotyledons was normal after a short, transient delay. Levels of representative enzymes of flavonoid biogenesis (phenylalanine ammonia-lyase, chalcone synthase) were somewhat increased rather than inhibited in the cotyledons of herbicide-treated, white-light-grown seedlings. The growth rate of hypocotyl and cotyledons was inhibited to the same extent in the herbicide-treated, white-light-grown seedling, although light inhibits growth of hypocotyls and promotes growth of cotyledons. Analysis of the data shows that photomorphogenesis of a herbicide-treated, white-light-grown seedling is normal, and is thus independent of plastid gene expression However, a 'factor' which coacts multiplicatively with phytochrome in determining the growth rate of the organs seems to originate from the plastids. Biogenesis of anthocyanin and synthesis of major enzymes of the flavonoid pathway are not affected adversely by a photooxidative elimination of plastid gene expression.

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