The Assembly of Ribosomes in Eukaryotes

Abstract

INTRODUCTION
This review is primarily concerned with the assembly of ribosomes, that is, the interaction of RNA and protein leading to the production of a functional ribosomal subunit. Yet this interaction has such a profound influence on the synthesis of ribosomal precursor RNA and its maturation to ribosomal RNA (rRNA), that it will be necessary to consider nearly every aspect of ribosome structure and biosynthesis to some degree. Fortunately a number of topics are discussed in greater detail in this volume. Reeder describes the organization of the genes for rRNA, and Maden, Salim and Robertson discuss the chemical relationship between ribosomal precursor RNA and rRNA.

Recent comprehensive reviews have been published by Maden (1971) and by Craig (1973).

COMPONENTS OF THE RIBOSOME
Ribosomal RNA
It now appears that nearly all RNA in eukaryotic cells is produced by the cleavage of longer transcripts (reviewed by Weinberg 1973). This concept was derived first from the observations on the formation of ribosomal RNA in mammalian cells from large ribosomal precursor RNA. The reader is referred to several recent reviews for the evidence demonstrating the pathways of maturation for various species (Craig 1973; Maden 1971; Attardi and Amaldi 1970). Elsewhere in this volume Maden, Salim and Robertson describe more recent work in this area.

In summary, the pathway is similar in all eukaryotes, from the mammals to the fungi (see Figure 1). A single transcript is formed in the nucleolus by a specific polymerase (reviewed by Jacob 1973). This is methylated, either during (Greenberg...