The mutant line albostrians of barley contains white plastids, devoid of ribosomes. In spite of this drastic defect the plastids contain normal amounts of DNA, and the restriction patterns of plastid DNA from white leaves and from green leaves are identical. Obviously, plastid DNA replication depends exclusively on the activity of nuclear genes. A nuclear-gene encoded RNA polymerase transcribes the genome of ribosome-deficient plastids. There are differences between white mutant plastids and green chloroplasts with respect to quantity, stability and processing of certain transcripts. Transfer RNA(Glu) is among those gene products which are not detectable in white, mutant plastids. This tRNA is not only essential for protein synthesis, it is also involved in the very first step of tetrapyrrole (including heme) and chlorophyll biosynthesis. It remains to be investigated where the heme-groups present in proteins of white tissue derive from. All other steps of tetrapyrrole and chlorophyll biosynthesis are under the control of nuclear genes. These genes seem to be (normally) active since feeding of delta-ALA to white leaves results in the synthesis of protochlorophyllide. The nuclear gene encoding the glutamate semialdehyde transferase (catalyzing an early step of tetrapyrrole synthesis leading to delta-ALA) shows an enhanced level of transcript accumulation in white as compared to green leaves. In contrast, nuclear genes encoding components of the photosynthetic apparatus including the enzymes of the Calvin cycle, show extremely low transcriptional activity. A plastid-derived signal chain has been proposed which triggers the activity of nuclear genes in accordance with the developmental state (''green'' or ''white'') of the plastids. Ribosome-free plastids may serve as a model system for studies on the regulatory interactions between plastids and the other DNA containing compartments (nucleus and mitochondria) in cells of higher plants, and on the role the plastid and nuclear genome play in the control of plastid metabolism in non-green tissues.
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