Progress in the Structural Analysis of Mammalian 45S and Ribosomal RNA

Abstract

INTRODUCTION
Several features distinguish eukaryotic from prokaryotic ribosomes with respect to both their structure and their formation. Eukaryotic ribosomes are larger than prokaryotic ones, their RNA is more highly methylated, and their formation takes place in a specialized organelle, the nucleolus, and involves transcription of a single large RNA precursor molecule (45S RNA in mammalian cells) (Maden 1971). The latter contains, in addition to the two ribosomal RNA sequences, considerable stretches of nonconserved material that are eliminated during maturation.

For a clearer insight into these phenomena, detailed chemical information on rRNA and its precursors is essential. The work described here has been directed towards this end. It relates in particular to two of the distinctive eukaryotic features mentioned above: rRNA methylation and the extensive nonconserved sequences in the precursor molecules. The work was carried out with HeLa cells. However, a comparable analysis has been carried out for methylation on yeast rRNA (Klootwijk and Planta 1973), and the results obtained, though different in detail, were strikingly similar in several general respects.

Methylation Patterns of HeLa Cell rRNA and Precursors
Sequence analysis of the methylated regions within rRNA and its precursors promised to be interesting in light of several previous findings with HeLa cells. These earlier (pre–1970) findings may be summarized as follows:Nearly all methylation occurs rapidly in the nucleolus, commencing on nascent 45S RNA (Greenberg and Penman 1966; Zimmerman and Holler 1967). There appeared originally to be one exception, late methylation of dimethyl A on 18S RNA (Zimmerman...