Assembly of Ribosomal Subunits

The reconstitution of active E. coli 50 S subunits, in contrast to that of 50 S particles from B. stearothermophilus (Nomura and Erdmann, 1970), requires a two-step incubation procedure (Nierhaus and Dohme, 1974 Dohme and Nierhaus, 1976). The assembly process occurs in four steps from 23 S RNA to 50 S particles, leading to formation of 33 S, 41 S, and 48 S intermediates. The step from 33 S to 41 S consists of a compact folding of the 33 S intermediate, without addition to any protein component. This drastic conformational change has been demonstrated by biochemical and electron-microscopic studies (Sieber and Nierhaus, 1978 Sieber et al, 1980 Nierhaus, 1982). Kinetic analyses performed at 

There are at least two assembly domains, namely the L20 domain and the L15 domain, in the 50 S assembly map (Fig. 15). Proteins within the L20 domain are essential for the assembly but not for the function of the 50 S subunit whereas those in the L15 domain are functionally important proteins whose assembly occurs at a late state. As with the 30 S subunit, the assembly map of the 50 S subunit (Rohl and Nierhaus, 1982) not only reflects the assembly dependence but also the topographical relationship of the proteins within the ribosomal particle. This conclusion is supported by a good correspondence between the assembly map on the one hand, and results from cross-linking studies and from the sequential removal of proteins from the particle by LiCl on the other hand. There is also a correlation between the interdependence of proteins during the assembly process and the arrangement of their genes on the E. coli chromosome (Rbhl et al., 1982). 

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