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Thursday, March 26, 2009

How Does Streptomycin Work?

Streptomycin is a powerful antibiotic that inhibits growth of bacteria while having very little effect on eukaryotes. It blocks protein synthesis by binding to the bacterial ribosome.

Let's review the steps of protein synthesis. The three steps are initiation, where the ribosome and factors are assembled at the start codon on the messenger RNA (mRNA); elongation, where the polypeptide chain elongates as the translation machinery moves along the mRNA; and termination, where the assembly falls apart and the completed polypeptide chain is release.

During the elongation phase there are three sites at the interface between the ribosome and the mRNA where transfer RNAs (tRNAs) can bind. The P site holds the peptidyl-tRNA molecule. The growing polypeptide chain is bound to the tRNA molecule that contributed the last amino acid. The A site binds the incoming aminoacylated tRNA molecule. The anticodon of this aa-tRNA is complementary to the mRNA codon located in the A site. Insertion of the correct aa-tRNA is mediated by elongation factor Tu (in bacteria)..

Formation of the peptide bond then occurs in a reaction catalyzed by peptidyl transferase—an enzymatic activity of the ribosomal RNA in the ribosome. When this happens the growing peptide chain (grey balls) is transferred to the tRNA that was in the A site.

The next step is to shift the ribosome relative to the mRNA bringing the peptidyl-tRNA molecule into position in the P site and freeing up the A site for a new amimoacyl tRNA to bind. This step is called translocation and it is mediated by elongation factor G (EF-G). During the translocation the uncharged tRNA is temporarily moved to the exit site (E site) before being released.

Streptomycin inhibits the translocation step by binding to the small subunit ribosomal RNA and blocking the activity of EF-G.

Here's a picture (below) of what the bacterial ribosome looks like. Most of it is RNA (yellow chain) folded into a complex conformation. The three dimensional structure is stabilized by a number of small proteins (orange + blue) bound to the outer surface of the RNA. One of these proteins is S12, located in the grove where mRNA binds to the ribosome. S12 stabilizes the RNA three-dimensional structure to which streptomycin binds.

Bacteria rapidly develop resistance to streptomycin, which explains why it isn't as effective today as it was when it was first introduced in the 1940s. One of the common resistance mutations affects ribosomal protein S12. The mutant protein is able to maintain the proper RNA conformation in the presence of streptomycin and this allow translocation to proceed.

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