ROLE OF POLYAMINES IN THE BINDING OF INITIATOR TRANSFER-RNA TO THE 70S RIBOSOMES OF EXTREME THERMOPHILIC BACTERIUM CALDEROBACTERIUM-HYDROGENOPHILUM

Slowly cooled cells of an extreme thermophilic eubacterium Calderobact erium hydrogenophilum possess ribosomes with weakly associated subunit s. These ribosomal subunits are capable of association to 70S ribosome s either at higher Mg2+ concentrations (30-40 mM) or at 4-10 mM Mg2+ a nd in the presence of polyamines. The contribution of 30S and 50S subu nits to the hydrodynamic stability of ribosomes was examined by formin g hybrid 30S-50S couples from C. hydrogenophilum and Escherichia coli. At lower Mg2+ (4-10 mM) heterogeneous subunits containing 30S E. coli and 50S C. hydrogenophilum and homogeneous subunits of the thermophil ic bacterium associated only in the presence of polyamines. Ribosomal subunits associated at 30 mM Mg2+ lose thermal stability and activity concerning poly(AUG)-dependent binding of f[H-3]Met-tRNA to the P-site on 70S ribosomes or translation of poly(UG). Poly(AUG), deacylated-tR NA or initiator-tRNA have no valuable effect on association of 30S and 50S subunits. Protein synthesis initiation factor IF3 of C. hydrogeno philum prevents association of ribosomal subunits to 70S ribosomes at physiological temperature (70 degrees C). The factor also stimulates d issociation of 70S ribosomes of E. coli at 37 degrees C. The codon-spe cific binding of f[H-3]Met-tRNA to homogeneous 70S ribosomes of C. hyd rogenophilum at 70 degrees C is dependent on the presence of initiatio n factors and concentrations of tri-pentaamines. However, excess of po lyamines inhibited the reaction. Our results indicate that tri-pentaam ines enhance conformational stability of 70S initiation complex at ele vated temperatures.