Volume 1

Gene expression / Benjamin Lewin.

  • Lewin, Benjamin.
Date:
1974-1977
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Gene expression / Benjamin Lewin. Source: Wellcome Collection.

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    sequence of nucleotides of the gene into the sequence of amino acids in its protein. However, this transfer does not take place directly on DNA itself, but utilises as an intermediate in information transfer an RNA copy of the gene. One of the concepts which arose early in ideas about protein synthesis was that amino acids might be assembled into protein on a nucleic acid template carrying the appropriate sequences of trinucleotides (see Crick, 1958). This template is now known as messenger RNA (mRNA) and is a molecule of (single stranded) RNA which bears an exact copy of the nucleotide sequence of the gene(s) on DNA specifying the protein(s) for which it codes (reviewed by Lipmann, 1963). The Iranscription of messenger RNA takes place from one strand of the duplex comprising the genes which it represents, so that its sequence is complementary to one strand of the DNA and identical with the other. The processes by which mRNA directs the formation of proteins are known as translation and require the participation of the other RNA con taining components of the protein synthesis apparatus, ribosomes and transfer RNA. Messenger RNA represents only a small proportion of cellular RNA, so that it has proved difficult to characterize. It was first identified in bacterial cells infected with bacteriophages as an RNA component specified by the invading phage. In phage infected cells, protein synthesis not accompanied by net synthesis of RNA, but as Volkin and Astrachan (1957) observed, a minor fraction—about 3%—of the RNA is both rapidly synthesised and rapidly degraded, as judged by its rate of incorporation of a P 32 label. The base composition of this RNA in E.coli cells infected with either phage T2 or phage T7 reflects that of the phage rather than the cellular DNA. This suggests that the infecting phage genome directs the production of phage specific proteins through the mediation of some unstable species of RNA molecule. More direct evidence for the production of unstable messenger RNAs by phage DNA was provided by the work of Brenner, Jacob and Meselson (1961), who found that a new RNA with a relatively rapid turnover is syn thesised in E.coli cells after infection with phage T2. This RNA has a nucleotide composition corresponding to that of the phage DNA and sediments in the range 14S-16S. Although sedimentation coefficients provide only imprecise estimates of RNA length, this should be the right order of magnitude to code for a long polypeptide chain. The RNA becomes associated with the pre existing ribosomes of the bacterial cell and is continually replaced. At the same time, Gros et al. (1961) detected a comparable species of RNA in uninfected E.coli which becomes pulse labelled and associates with the ribosomes engaged in protein synthesis. Experiments involving the hybridization of messenger RNA to denatured DNA have shown that, as expected, it constitutes a copy of the nucleotide sequences of DNA. Experiments with the rather crude techniques then available were used by Hall and Spiegelman (1961) to show that T2 mRNA