Biosynthesis of Organelle Ribosomes

Abstract

INTRODUCTION Compared with the status in prokaryotic and eukaryotic ribosomes, knowledge of the biosynthesis of organelle ribosomes is in early development. Information concerning the protein components of these ribosomes is still incomplete. Since animal cells contain two potential sites for structural genes and for protein synthesis, and plant cells contain three, the question of biosynthetic origin and genetic specification of the ribosomal proteins is potentially complex. The RNA components are better characterized, and there is evidence that in each case they are transcription products of the organelle DNA. Yet little is known concerning RNA precursor molecules, and the questions of control of organelle ribosome assembly or coordination of biosynthesis of the structural components have not yet been approached. This review will attempt to summarize current knowledge of these topics and to point out areas that require development. STRUCTURE AND PROPERTIES OF ORGANELLE RIBOSOMES It has been known for some time that mitochondria and chloroplasts both contain ribosomes that have properties distinctly different from those of their counterparts in the cell sap. The structures and functions of these ribosomes have been extensively reviewed in several recent articles (Borst and Grivell 1971; Kuntzel 1971; Boulter, Ellis and Yarwood 1972; Borst 1972; Kroon, Agsteribbe and De Vries 1972; Mahler 1973). Chloroplast ribosomes were first isolated from spinach by Lyttleton in 1962, and since then they have been isolated and characterized from several higher plants and from green algae (Boulter, Ellis and Yarwood 1972). In contrast to the 80S ribosomes of the cell sap,...