Early Assembly Proteins of the Large Ribosomal Subunit of the Thermophilic Archaebacterium Sulfolobus

Studies of ribosome structure in thermophilic archaebacteria may provide valuable information on (i) the mechanisms involved in the stabilization of nucleic acid-protein complexes at high temperatures and (ii) the degree of evolutionary conservation of the ribosomal components in the primary kingdoms of cell descent. In this work we investigate certain aspects of RNAIprotein interaction within the large ribosomal subunits of the extremely thermophilic archaebacterium Sulfolobus solfataricus. The ribosomal proteins involved in the early reactions leading to in vitro particle assembly have been identified; it is shown that they can interact with the RNA in a temperatureindependent fashion, forming a thermally stable “core” particle that can subsequently be converted into complete 50 S ribosomes. Among the protein components of the core particle, those capable of independently binding to 23 and 5 S RNA species have also been identified. Finally, we show that the early assembly proteins of Sulfolobus large ribosomal subunits are able to interact cooperatively with 23 S RNAs from other archaebacteria or from eubacteria, thereby suggesting that RNAIprotein recognition sites are largely conserved within prokaryotic ribosomes. By contrast, no specific binding of the archaebacterial proteins to