Pactamycin binding to Escherichia coli ribosomes

Resistance to pactamycin in clones of Streptomyces lividans containing DNA from pactamycin-producing Streptomyces pactum

A pactamycin (Pc)-resistance determinant (pct) from Streptomyces pactum has been isolated on a 4.9-kb KpnI fragment. The original construct involving plasmid pIJ702 was highly unstable in Streptomyces lividans, leading to deletion of the pct gene from the vector. Subcloning of pct into an alternative vector (pOJ160) led to the generation of a more stable clone which possessed Pc-resistant ribosomes, and reconstitution analysis established that 16S rRNA was responsible for such resistance. Post-transcriptional modification of rRNA is probably the mechanism of resistance since the cloned DNA fragment did not appear to encode 16S rRNA.

Photolabeling of protein components in the pactamycin binding site of rat liver ribosomes

The antitumoral and antibacterial drug pactamycin can be radioactively labeled by iodination without loss of biological activity. Using the labeled pactamycin, the ribosomal binding site of the drug on rat liver ribosomes has been studied by affinity labeling techniques taking advantage of the photoreactive acetophenone group present in the molecule. When 40 S ribosomal subunits are labeled, one major spot of radioactivity is found associated to protein S25. In addition, weaker spots related to proteins S14/15, S10, S17 and S7 can also be detected in the autoradiogram of the two-dimensional gel slab. Since pactamycin inhibits protein synthesis initiation, the proteins forming its binding site must be related to some step of this process. By comparison with results from pactamycin affinity labeling of Escherichia coli ribosomes (Tejedor, F., Amils, R. and Ballesta, J.P.G. (1985) Biochemistry 24, 3667-3672) these proteins could lie in the mRNA and initiation factors binding region of the rat liver ribosome.

Ribosomal function and its inhibition by antibiotics in prokaryotes

Most of the known antibiotics act at the level of protein biosynthesis probably due to the extraordinary complexity of the translation machinery which can be interfered with at many points. At first a survey is given of our present knowledge covering the structure and function of the prokaryotic ribosome. The most important antibiotics acting at the translational level are integrated into this network of data. The binding sites and the inhibition mechanisms of the drugs, together with the ribosomal components altered in resistant mutants are described. Finally, the points of interference with the translational machinery are indicated in an extended scheme of ribosomal functions.