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Figure 7.8. Scheme of the disassembly of the 30S ribosomal subunit achieved by high salt concentration (e.g., by an increased concentration of LiCl in the presence of 5 mM MgCl2; see the text).

The first proteins to bind to ribosomal RNA in the course of self-assembly are the core proteins which are capable of binding to RNA independently of each other. In the case of E. coli 30S subunits, these are proteins S4, S7, S8, S15, S17, and S20. Protein S16 binds along with these. The addition of the six proteins S4, S7, S8, S15, S16, and S17 (step I in the scheme of Fig. 7.9) is a prerequisite for the transition of the intermediate ribonucleoprotein from a less compact to a more compact state (step II in Fig. 7.9). Apparently, it is this transition that requires an elevated temperature during self-assembly. As a result of this transition, the 16S RNA almost reaches the maximal compact state of its overall folding which is characteristic of this RNA within the mature 30S ribosomal subunit.

Proteins S20, S6-S18, S5, S9, S11, S12, S13, and S19 may enter the complex concurrently with the aforementioned proteins, even before the transition of the complex to a mare compact state. However, Fig. 7.9, which shows the sequence and interdependence of protein binding in the course of E. coli 30S subunit reconstitution, presents the incorporation of these proteins into the ribonucleoprotein as step III of self-assembly, since these proteins, even when bound, are not strictly necessary for the transition into a more compact state and can bind to the complex after this transition. Moreover, as shown in Fig. 7.9, the binding of most proteins at this stage of self-assembly depends on the presence of the set of previously bound proteins. For example, the binding of proteins S9 and S19 requires that protein S7 be bound with RNA. The attachment of proteins S6:S18 depends on the presence of protein S15 and, probably to a lesser extent, of protein S8. The binding of protein S11 requires the presence of proteins S6:S18. The attachment of protein S5 is induced by proteins S8 and S16. (It should be emphasized once again that steps II and III shown in

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