Purification of ribosomal protein S1 and physical tests of its homogeneity

Localization of the elongation factor Tu binding site on Escherichia coli ribosomes

Fluorescent techniques were used to study binding of peptide elongation factor Tu (EF-Tu) to Escherichia coli ribosomes and to determine the distances of the bound factor to points on the ribosome. Thermus thermophilus EF-Tu was labeled with 3-(4-maleimidylphenyl)-4-methyl-7-(diethyl-amino)coumarin (CPM) without loss of activity. In the presence of Phe-tRNA and a nonhydrolyzable analogue of GTP, 70S ribosomes bind the CPM-EF-Tu [Kb = (3 +/- 1.2) X 10(6) M-1] causing a decrease of CPM fluorescence. Binding of CPM-EF-Tu to 50S subunits was at least 1 order of magnitude lower than with 70S ribosomes, and binding to 30S subunits could not be detected. Reconstituted 70S ribosomes containing either S1 labeled with fluoresceinmaleimide or ribosomal RNAs labeled at their 3' ends with fluorescein thiosemicarbazide were used for energy transfer from CPM-EF-Tu. The distances between CPM-EF-Tu bound to the ribosomes and the 3' ends of 16S RNA, 5S RNA, 23S RNA, and the closest sulfhydryl group of S1 were calculated to be 82, 70, 73, and 62-68 A, respectively.

Comparison of the misreading induced by streptomycin and neomycin

In a poly(U)-programmed translation system, neomycin stimulates the misincorporation of tyrosine and of serine which, according to Thompson and Stone (Thompson, R.C. and Stone, P.J. (1977) Proc. Natl. Acad. Sci. USA. 74, 198-202), are normally rejected at an initial discrimination step during the binding of charged tRNAs to the ribosome. In contrast, streptomycin favors the misincorporation of isoleucine which is normally rejected at a subsequent GTP-dependent discrimination step, the so-called proofreading step. The labeling of the ribosome with N-ethylmaleimide mimics the effect of streptomycin in that it stimulates the misincorporation of isoleucine but not of tyrosine or serine. This effect is correlated with the labeling of protein S18 but not with that of protein S1. These observations indicate that the sulfhydryl group of protein S18 is located within a ribosomal domain involved in the proofreading control of tRNA selection. Taking into account our previous results that streptomycin and neomycin perturb ribosomal areas around the sulfhydryl groups of proteins S18 and S1, respectively, we suggest that these antibiotics induce misreading by different mechanisms which are linked to such perturbations.

Fragments of ribosomal protein S1 and its mutant form m1-S1. Localization of nucleic-acid-binding domain in the middle region of S1

Escherichia coli ribosomal protein S1 and its mutant, shorter, form m1-S1 were cleaved at internal methionyl residues to yield, respectively, six and five fragments of Mr ranging from 1000 to 24000. Methods are described to isolate the fragments in pure form. Four of the fragments (designated F2a, F2b, F3 and F4) contain between 86 and 215 amino acids and are therefore as large as other ribosomal proteins. Fragment F2a, derived from the N-terminal region, has previously been shown to contain the major ribosome binding domain of S1 [S. Giorginis and A. R. Subramanian (1980) J. Mol. Biol. 141, 393--408]. Here we show that the RNA binding domain of S1 is essentially contained in F3 derived from the middle region of S1 and carrying the nonreactive--SH group. The reactive--SH group of S1, whose activity is modified by ligand binding, was localized in F2b, a fragment with little RNA binding capacity. The characteristic RNA binding domain and a weak ribosome binding domain of S1 have previously been localized in the large trypsin-resistent core S1-F1 [T. Suryanarayana and A. R. Subramanian (1979) J. Mol. Biol. 127, 41--54]. Together these data indicate that two of the key functional domain of S1 are located in two regions of the molecule separated by an open, exposed segment. The present study also revealed that the nonreactive--SH group of S1 becomes reactive in m1-S1 by the loss of the remote C-terminal region in the latter.