1. Introduction

6.1 Introduction


An mRNA contains a series of codons that interact with the anticodons of aminoacyl-tRNAs so that a corresponding series of amino acids is incorporated into a polypeptide chain. The ribosome provides the environment for controlling the interaction between mRNA and aminoacyl-tRNA. The ribosome behaves like a small migrating factory that travels along the template engaging in rapid cycles of peptide bond synthesis. Aminoacyl-tRNAs shoot in and out of the particle at a fearsome rate, depositing amino acids; and elongation factors cyclically associate and dissociate. Together with its accessory factors, the ribosome provides the full range of activities required for all the steps of protein synthesis.


The ribosome possesses several active centers, each of which is constructed from a particular group of proteins associated with a region of ribosomal RNA. The active centers require the direct participation of rRNA in a structural or even catalytic role. Some catalytic functions require individual proteins, but none of the activities can be reproduced by isolated proteins or groups of proteins; they function only in the context of the ribosome.


Two types of information are important in analyzing the ribosome. Mutations implicate particular ribosomal proteins or bases in rRNA in participating in particular reactions. Structural analysis, including direct modification of components of the ribosome and comparisons to identify conserved features in rRNA, identifies the physical locations of components involved in particular functions.




Figure 6.1 Ribosomes are large ribonucleoprotein particles that contain more RNA than protein and dissociate into large and small subunits.

The basic form of the ribosome is conserved, but there are appreciable variations in the overall size and proportions of RNA and protein in the ribosomes of bacteria, eukaryotic cytoplasm, and organelles. Figure 6.1 compares the components of bacterial and mammalian ribosomes. Both are ribonucleoprotein particles that contain more RNA than protein. The ribosomal proteins are known as r-proteins.


All ribosomes in a cell are identical. A ribosome always consists of two subunits, each of which contains a major rRNA and a number of small proteins. The large subunit may also contain smaller RNA(s). In E. coli, the small (30S) subunit consists of the 16S rRNA and 21 proteins. The large (50S) subunit contains 23S rRNA, the small 5S RNA, and 31 proteins. With the exception of one protein present at four copies per ribosome, there is one copy of each protein.


The ribosomes of higher eukaryotic cytoplasm are larger than those of bacteria. The total content of both RNA and protein is greater; the major RNA molecules are longer (called 18S and 28S rRNAs), and there are more proteins. Probably most or all of the proteins are present in stoichiometric amounts. RNA is still the predominant component by mass.


Organelle ribosomes are distinct from the ribosomes of the cytosol, and take varied forms. In some cases, they are almost the size of bacterial ribosomes and have 70% RNA; in other cases, they are only 60S and have <30% RNA.




Figure 6.2 Electron microscopic images of bacterial ribosomes and subunits reveal their shapes. Photographs kindly provided by James Lake.

Electron micrographs of subunits and complete bacterial ribosomes are shown in Figure 6.2, together with models in the corresponding orientation. The complete 70S ribosome has an asymmetric construction. The partition between the head and body of the small subunit is aligned with the notch of the large subunit, so that the platform of the small subunit fits into the large subunit. There is a cavity between the subunits which contains some of the important sites (for review see 430, 433, 434).


The RNAs constitute the major part of the mass of the bacterial ribosome. Their presence is pervasive, and probably most or all of the ribosomal proteins actually contact rRNA. So the major rRNAs form what is sometimes thought of as the backbone of each subunit, a continuous thread whose presence dominates the structure, and which determines the positions of the ribosomal proteins.


Reviews
430: Wittman, H. G. (1983). Architecture of prokaryotic ribosomes. Ann. Rev. Biochem 52, 35-65.
434: Hill, W. E. et al. (1990). The Ribosome. American Society for Microbiology, Washington DC.

Research
433: Noller, H. F. and Nomura, M. (1987). Ribosomes. In E. coli and S. typhimurium, Ed. F. C. Neidhardt, American Society for Microbiology, Washington DC , -.





Genes VII
Genes VII
ISBN: B000R0CSVM
EAN: N/A
Year: 2005
Pages: 382

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