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.

    143/668 (page 121)
    of 5S rRNA is common to both bacterial and eucaryotic ribosomes and in bacteria is inserted at a late stage of ribosome assembly (see page 129). Attach ment is permanent, for Kaempfer and Meselson (1968) found that once 5S rRNA has been inserted in the 50S subunit it cannot exchange with free 5S rRNA or with other ribosomes. The 5S rRNA molecule is not released by some treatments which dissociate many proteins from the ribosome, but is released by unfolding the ribosome with EDTA (see Hosokawa, 1970). This suggests that it may be located within the structure of the 50S subunit. That 5S rRNA is bound to particular ribosome protein(s) is suggested by the observation of Blobel (1971) that EDTA releases a complex of 5S rRNA with one protein from the ribosomes of rat liver or rabbit reticulocytes. Gray et al. (1972) found that 5S rRNA can form a complex with 23S rRNA in the presence of two or three specific proteins of the 50S subunit of E.coli. The presence of 5S rRNA is essential for the activity of 50S subunits but we do not know whether its role in the subunit is structural or connected with some particular reaction of protein synthesis. Some 5S rRNAs have been sequenced, but the primary structures do not suggest any obvious conformation for the molecule. E.coli 5S rRNA has no minor bases and Brownlee, Sanger and Barrell (1967, 1968) found that both the strains of E.coli which they used contain two species of 5S rRNA, one of which is the same in each strain and the second of which is related to it by a single base substitution. There must therefore be more than one gene respon sible for its synthesis; since there are two types of 5S rRNA and only one 5S rRNA molecule on each ribosome, this implies the presence of two types of ribosome in the cell. But this is unlikely to have any biological significance. Two base sequences—one of eight and one of ten residues—are repeated in the molecule; and when the structure is written so that these are aligned there is enough homology to suggest that the 5S rRNA molecule may have arisen by gene duplication. The 5S rRNA from human tumour KB cell ribosomes has been sequenced by Forget and Weissman (1969) and, in spite of its similar size, has little homology with the bacterial 5S rRNA. The 5S rRNA isolated by Williamson and Brownlee (1969) from each of two mouse cell lines in culture shows the same T1 and pancreatic ribonuclease digest fingerprints as does that from KB cells; this suggests that its sequence is probably the same in both man and mouse, although the presence of small differences cannot be excluded. Averner and Pace (1972) found the same oligonucleotides in a digest of 5S rRNA of the marsupial rat kangaroo. The sequence of 5S rRNA may therefore be the same in all mammals. A very large number of genes code for 5S rRNA in the eucaryotic genome (see chapter five of volume 2), so the identification of only one sequence in each species implies that all the 5S genes must be identical or at least so closely related that their oligonucleotide digest patterns remain the same. There must therefore be considerable evolutionary restraint on variation in