Interconversion of tight and loose couple 50 S ribosomes and translocation in protein synthesis

On incubation of 50 S ribosomes, isolated from either tight couple (TC) or loose couple (LC) 70 S ribosomes, with elongation factor G (EG-G) and guanosine 5'-triphosphate, a mixture of TC and LC 50 S ribosomes is formed. There is almost complete conversion of LC 50 S ribosomes to TC 50 S ribosomes on treatment with EF-G, GTP, and fusidic acid. Similarly, TC 50 S ribosomes are converted to LC 50 S ribosomes, although partially, by treatment with EF-G and a GTP analogue like guanyl-5'-yl methylenediphosphate (GMP-P(CH2)P) or guanyl-5'-yl imidodiphosphate (GMP-P(NH)P) and including a polymer of 5'-uridylic acid (poly(U] in the incubation mixture. Furthermore, LC 23 S RNA isolated from LC 50 S ribosomes is converted to TC 23 S RNA on heat treatment, but similar treatment does not affect TC 23 S RNA. The interconversion was followed by several physical and biological characteristics of TC and LC 50 S ribosomes, like association capacities with 30 S ribosomes before and after kethoxal treatment, susceptibility to RNase I and polyphenylalanine-synthesizing capacity in association with 30 S ribosomes, as well as thermal denaturation profiles, circular dichroic spectra, and association capacity of isolated 23 S RNAs. These data strongly support the proposition that TC and LC 50 S ribosomes are the products of translocation during protein synthesis. The conformational change of 23 S RNA induced by EF-G and GTP is most probably responsible for the interconversion, and L7/L12 proteins play an important role in the process. A two-site model based on kethoxal data has also been proposed to explain the tightness and looseness of 70 S couples.