Segmental flexibility of ribosomal protein S1 bound to ribosomes and Q beta-replicase

Relaxation time, interthiol distance, and mechanism of action of ribosomal protein S1

The two sulfhydryl groups of ribosomal protein S1 from Escherichia coli have been labeled with fluorescent maleimides and the distance between them has been determined by nonradiative energy transfer. This distance was found to be approximately 27 A for both free S1 and S1 bound to 30 S subunits. This value probably represents an upper limit. The position of the fluorescence emission maximum indicates that both sulfhydryl groups are in a relatively hydrophobic environment. When poly(U) is added to labeled S1, either free or in 30 S subunits, the emission maximum shifts to the red by about 3 nm but without a detectable change in the interthiol distance. S1 labeled at one or both of its sulfhydryl groups retains most of its ability to enhance poly(U)-directed polyphenylalanine synthesis. About the same concentration of poly(U) is required to give the maximum shift in fluorescence as is required to give maximum polyphenylalanine synthesis, indicating that S1 binds poly(U) during translation. The peptide initiation inhibitor aurintricarboxylic acid almost completely quenches the fluorescence from either labeled sulfhydryl groups in S1 bound to ribosomes or free in solution. This quenching probably is due to energy transfer from the labeled sulfhydryls to bound aurintricarboxylic acid. Fluorescence anisotropy measurements indicated that the C-terminal domain of S1 is relatively rigid, but retains some independent movement when attached to ribosomes. The overall data are consistent with a model in which a region near the two sulfhydryl groups in the elongated C-terminal domain functions to sequester and bind mRNA to the ribosome during peptide synthesis.

The binding of ribosomal protein S1 to S1-depleted 30S and 70S ribosomes. A fluorescence anisotropy study of the effects of Mg2+

We have determined the equilibrium constants for the binding of AEDANS-labelled S1 to S1-depleted 30S and 70S ribosomes. For "tight" ribosomes, the association of S1 increases with the sixth power of Mg2+ concentration, but for 30S subunits and "loose" ribosomes, there is virtually no dependence of the association on Mg2+ over the same concentration range, 2-10 mM in Mg2+. The binding of S1 to 70S ribosomes at 10 mM Mg2+ is stabilized by 2 kcal/mol compared to the binding to 30S subunits. When intact S1 binds to tight ribosomes, the fluorescence anisotrophy is that for virtually complete rotational immobilization. The anisotropies vary considerably with the preparation and treatment of both S1 and ribosomes and these variations are detailed here. The results suggest the linkage of Mg2+-dependent conformational changes in the intact ribosomes, perhaps including rRNA, and the binding of S1.