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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    contents to the assembly map point to the conclusion that assembly both in vitro and in vivo involves a first step in which a small number of proteins—S4, S8, S20, S7, S15 (?), S16, S17 (?), S13 and perhaps S9, SI9 and S6—associate with 16S rRNA to form a precursor particle. It is possible that this may be the only precursor, to which the remaining 30S proteins are then added; or two steps may remain in maturation, forming first a 26S and then a 30S particle. Organization of Proteins in the 30S Submit The order of assembly and the components found in precursors may indicate in an approximate way the topology of the mature subunit. Those proteins which are added first during assembly tend to be those located in the interior of the ribosome and the proteins added later appear to be present at the surface of the ribosome. This idea is supported by the identification of the proteins which are split from 30S subunits by caesium or lithium ions. Table 4.2 shows that both sets of split proteins are similar, although caesium ions are less effective, probably because of their larger ionic radius. The proteins which are split tend to be those which are absent from the precursor particles and are located late in the assembly map. The precursor particles may therefore contain a nucleus of proteins which form part of the interior of the mature subunit when later proteins are added to form the exterior. Crude maps of the 30S subunit can on this basis be drawn from the relation ships of its proteins and as figure 4.7 shows the results are similar according to criteria of either structure or assembly. Proteins have been characterized by their resistance to extraction with salt : the clear outer circle of the salt structure map represents proteins which are removed by CsCl; the shaded intermediate circle shows the additional proteins removed by molar LiCl; and the dark inner nucleus comprises the proteins resistant to both treatments. A very similar arrangement is found in the precursor particle map, in which the inner core consists of proteins found both in the 21S precursors accumulated by the sad and spectinomycin-resistant mutants and in the RI particles. The inter mediate circle contains the proteins found in some but not all of these particles ; and the outer circle marks the proteins which are never found in any of these particles. Another way to study the topology of the ribosome is to treat 30S subunits with reagents which modify or degrade proteins. Those proteins which are essential for assembly and are found in the precursors tend to be inaccessible to reagents such as iodoacetate or methoxy-nitropone. As table 4.3 shows, the proteins which Chang and Flaks (1971) and Craven and Gupta (1970) found are protected against degradation by trypsin include those present in the precursors. There is an approximately inverse relationship between the sequence of addition of proteins during ribosome assembly and their relative