Magnesium complexes ribosomes

Streptomycin has a complex action on bacteria the general background is given in Section 4.1 (p. 126)5 and the unusual penetration of the plasma membrane is described in Section 14.2. What is of interest here is that streptomycin displaces magnesium from ribosomes as shown in Fig. 11.7. This effect seems to be a case of competition among cations rather than a metal-binding effect. 

The glycoside/aminoglycoside antibiotics, like the macrolides, exert a bacteriostatic effect due to selective inhibition of bacterial protein synthesis, with the exception of novobiocin (26). The compounds neomycin (27), spectinomycin (28) and streptomycin (29) bind selectively to the smaller bacterial 30S ribosomal subunit, whilst lincomycin (30) binds to the larger 50S ribosomal subunit (cf. macrolides). Apramycin (31) has ribosomal binding properties, but the exact site is uncertain (B-81MI10802). Novobiocin (26) can inhibit nucleic acid synthesis, and also complexes magnesium ion, which is essential for cell wall stability. 

EF-Tu is a GTPase that positions aminoacyl-tRNA complexes within the A site of prokaryotic ribosomes during protein synthesis. The binding of a GTP molecule (red) by EF-Tu causes domain conformation changes (not shown) that result in the creation of a binding cleft for an aminoacyl-tRNA complex. GTP hydrolysis causes domains 1 and 2 to move apart so that the aminoacyl-tRNA (not shown) is released. (A magnesium ion is illusttated with a yellow sphere.)... 

In estimating the extent of covalent binding to the ribosome, assays must be employed that can clearly distinguish between reversibly and irreversibly bound material. Toward this end, the following methods were used (a) treatment with buffers of low magnesium concentration that would completely dissociate reversible binding complexes, and (b)... 

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