10 The Ribosome and Its Synthesis

Abstract

I. INTRODUCTION
The ribosome can be considered the blue-collar worker of the cell, faithfully carrying out the designs provided by the structural genes. In this role, the ribosome is central to the growth and maintenance of the cell, and, in turn, many of the functions of the cell are involved in ensuring a steady and regulated supply of ribosomes. Studies of the biosynthesis of ribosomes and its regulation provide us with an entrée into many of the key questions of molecular and cellular biology. Expression of ribosomal RNA (rRNA) and protein genes involves most of the known machinery and processes of eukaryotic gene expression: transcription by RNA polymerases I, II, and III; posttranscriptional modification and processing of precursor RNAs; and translation of mRNAs. Efficient biosynthesis of ribosomes must involve regulation of these processes to produce equal amounts of rRNAs and ribosomal proteins in proportion to cellular growth rates. Consequently, studies of ribosome biosynthesis are closely intertwined with those of growth rate regulation. rRNA transcription and processing as well as ribosome assembly take place within a special organelle, the nucleolus; little is known about its structure and operation. The ribosomal proteins are synthesized in the cytoplasm and transported to the nucleus. The completed ribosomal subunits are transported from the nucleolus to the cytoplasm. Almost nothing is known about either of these processes. The processing of rRNA and the role of ribosomal proteins, nonribosomal proteins, and small nucleolar RNAs in this processing are similarly hazy. These are questions of fundamental importance for...