Cytoplasmic synthesis of plastid polypeptides may be controlled by plastid-synthesised RNA

Abstract

IT is generally agreed that some plastid polypeptides are coded by plastid DNA and the remainder by nuclear DNA, and that some plastid polypeptides are synthesised on plastid ribosomes and the remainder on cytoplasmic ribosomes^1,2. Studies on two mutant lines of barley (Hordeum vulgare L. ‘albostrians’ (M4205) and ‘Saskatoon’) have helped the understanding of the integration of the plastid and nucleocytoplasmic systems. In both lines a nuclear mutation is expressed as an irreversible mutation of the plastid DNA yielding pigment-deficient plastids whose segregation leads to white-striped leaves and later to pure white leaves and shoots^3,4. The white leaves do not contain plastid ribosomes and no protein synthesis can be detected in the plastids, although cytoplasmic ribosomes are present in normal amounts^5,6. Immunoelectrophoresis of extracts of white leaves has failed to detect either the plastid-synthesised large subunit or the cytoplasmically synthesised small subunit of ribulose-bisphosphate carboxylase/oxygenase, or coupling factor CF₁, which has three plastid-synthesised and two cytoplasmically synthesised subunits, or the cytoplasmically synthesised ferredoxin-NADP⁺ reductase^6,7. We report here that two cytoplasmically synthesised plastid enzymes⁸, phosphoribulokinase (EC 2.7.1.19) and D-glyceraldehyde-3-phosphate: NADP⁺ oxidoreductase (phosphorylating) (EC 1.2.1.13), occur in considerably reduced amounts in white leaves and deduce how the plastid may control the cytoplasmic synthesis of plastid polypeptides.