THE ROLE OF MEMBRANE-BOUND TRANSCRIPTION AND TRANSLATION SYSTEMS IN BIOGENESIS OF CHLOROPLAST MEMBRANES

Biochemical and electron-microscopic studies of the membrane system of pea chloroplasts at early stages of their light-induced biogenesis su ggest the direct involvement of transcription and translation mechanis ms linked to the primary membranes of etiochloroplasts, Eight sequenti al stages are observed: I) appearance of primary transcripts in the et iochloroplast membrane; II) maturation of the transcripts (splicing); III) assembly of ribosomal monomers; IV) formation of polysomes, their functioning, and incorporation of nascent polypeptides into membrane complexes; V) assembly of membrane complexes covering the polysomes an d formation of fret-like structures; VI) transformation of the thicken ings of fret-like structures into prothylakoids; VII) formation of gra na thylakoids (stages I-VII proceed in the primary membrane); VIII) co mpletion of biogenesis by conversion of thus modified primary membrane into a three-dimensional structure of the mature chloroplast membrane system with polyribosomes localized in the marginal area of the grana thylakoids. The principle of insertion of polyribosomes into membrane structures seems to be common for photosynthetic membranes determinin g their formation and evolutionary development as indicated by a simil ar organization of the membrane-bound translation system of the cyanob acterium Phormidium laminosum. It is suggested that a special type of membrane structure comprising translation machinery and coupling inter dependent mechanisms of protein synthesis and macroergic compound prod uction can be distinguished.