The assembly of the prokaryotic ribosome

Abstract

The ribosome contains two subunits; both of them consist of a few RNA molecules and a large number of different proteins. For example, the small subunit (30 S) from Escherichia coli ribosomes consists of one RNA molecule and 21 different proteins, and the large one of two RNA molecules and 32 proteins. All the components except one protein are present in one copy per ribosome. The in vivo assembly of these complicated structures occurs via distinct halting points reflected by the occurrence of precursor particles, two for the small and three for the large subunit. Both subunits can also be assembled in vitro from the purified components (total reconstitution). In the course of the in vitro assembly intermediate particles are formed which correspond nicely to the in vivo precursor particles. In spite of serious differences between in vivo and in vitro assembly the analysis of the latter has revealed many interesting details. The rate limiting steps of the subunit formation are conformational changes, one in the case of the small and two in the case of the large subunit. The components for the conformational change in the early assembly have been identified for both subunits. A comparison of the early assembly of proteins of the 50 S subunit with their RNA binding sites has led to the concept that the 50 S assembly in vivo starts on the partially synthesized RNA thread, and that the progress of RNA synthesis dictates the progress of 50 S assembly. Furthermore, assembly proteins have been identified which are essential for the early 50 S assembly but play no role for the late assembly or functions of the mature 50 S subunit. The protein-by-protein addition has been analysed in vitro, and has culminated in an “assembly map” for each subunit. The current assembly research concentrates on attempts to define the importance (hierarchy) of each ribosomal component with respect to the assembly process.