RIBOSOME-DEFICIENT PLASTIDS OF ALBOSTRIANS BARLEY - EXTREME REPRESENTATIVES OF NONPHOTOSYNTHETIC PLASTIDS

The mutant line albostrians of barley contains white plastids, devoid of ribosomes. In spite of this drastic defect the plastids contain nor mal amounts of DNA, and the restriction patterns of plastid DNA from w hite leaves and from green leaves are identical. Obviously, plastid DN A 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 plasti ds and green chloroplasts with respect to quantity, stability and proc essing of certain transcripts. Transfer RNA(Glu) is among those gene p roducts which are not detectable in white, mutant plastids. This tRNA is not only essential for protein synthesis, it is also involved in th e very first step of tetrapyrrole (including heme) and chlorophyll bio synthesis. It remains to be investigated where the heme-groups present in proteins of white tissue derive from. All other steps of tetrapyrr ole and chlorophyll biosynthesis are under the control of nuclear gene s. 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 (catalyzi ng 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 ph otosynthetic apparatus including the enzymes of the Calvin cycle, show extremely low transcriptional activity. A plastid-derived signal chai n has been proposed which triggers the activity of nuclear genes in ac cordance with the developmental state (''green'' or ''white'') of the plastids. Ribosome-free plastids may serve as a model system for studi es on the regulatory interactions between plastids and the other DNA c ontaining compartments (nucleus and mitochondria) in cells of higher p lants, and on the role the plastid and nuclear genome play in the cont rol of plastid metabolism in non-green tissues.