Probing of mRNA binding sites involved in interactions with rat liver ribosomes using poly(U) spin labeled at the ribose moiety

The interaction of rat liver ribosomes with poly(U), spin labeled (SL) at the 2'OH groups of ribose residues by N-(2,2,5,5-tetramethyl-3-carbonylpyrroline-1-oxyl)-imidazole, has been studied by electron spin resonance (ESR) spectroscopy. The ESR spectra demonstrate that SL-poly(U) with a modification of 1 spin label per 20 ribose residues binds to 80S ribosomes as well as to 40S subunits in a 1:1 stoichiometry at 12 mM MgCl2. The same result is found with highly modified poly(U) bearing 1 SL per 4 ribose residues. Addition of excessive amounts of unmodified poly(U) displaces bound SL-poly(U) from the ribosome which points to a competition for the same binding site at the ribosome. The biological activity of SL-poly(U) was tested with regard to trigger 80S ribosomes for binding of Phe-tRNAPhe and for poly(Phe) synthesis. SL-poly(U) bearing 1 SL group per 20 ribose residues directs the binding of only 50% of the amount of Phe-tRNAPhe bound to ribosomes in the presence of unmodified poly(U). When SL-poly(U) bearing 1 SL group per 4 ribose residues is used, this value drops to 25%. Poly(Phe) synthesis is even more impaired: In the presence of poly(U) bearing 1 SL-group per 20 ribose residues only about 30% of the amount of poly(Phe) coded by unmodified poly(U) are synthesized and in the presence of SL-poly(U) bearing 1 SL group per 4 ribose residues poly(Phe) synthesis is completely abolished. The results suggest that modification of the ribose moiety has only a relatively small influence on the binding of mRNA to ribosomes but causes substantial impairment of the mRNA function, whereas, as shown earlier (Ebert et al., Acta Biol. Med. Germ. 41, 431, 1982), modification of the base moiety of poly(U) (in a proportion of 1 SL per 30 bases) does not influence coding efficiency for poly(Phe) synthesis.