Question

What is the target of selection (in bacteria) by streptomycin?

Nucleotide synthesis

Ribosomal proteins involved in protein synthesis

RNA polymerase

Cell wall synthesis

Answer 1

Ans) Ribosomal proteins involved in protein synthesis

The prokaryotes (e.g. bacteria) consist of 70S ribosomes which are made up of one larger subunit called 50S and smaller subunit called 30S. The 50S subunit consists of 23S rRNA made up of 2904 Nucleotides, 5S r RNA composed of 120 Nucleotides long and 31 proteins. The 30S subunit has 16S rRNA (1542 Nucleotide long) & 21 proteins. Both 50S & 30S of 70S ribosome have three tRNA binding sites. A site or aminoacyl site to which charged aminoacylated tRNA binds, P site or peptidyl site is the site to which nascent polypeptide is attached and E site or exit site which binds to the deacylated tRNA. Both subunits of 70S ribosome make contact with the acylated charged tRNA.

In 1943 first time Streptomycin was isolated from the Streptomyces griseus and chemically it is an aminoglycoside. It is used to treat tuberculosis (1 μg/ml - 2 μg/m) caused by the Mycobacterium, brucellosis, plague and the vomiting and rashes on the body are most common side effects associated with the Streptomycin. It is not recommended to the pregnant woman and myasthenia gravis patients. Streptomycin inhibits protein synthesis in both gram +ve and gram –ve bacteria and It binds to the 16s rRNA of smaller 30S subunit & inhibit the binding of formyl-methionyl tRNA (charged/ acylated tRNA) to A site on 30S subunit thereby prevent the initiation of protein synthesis in bacteria which interne leads to bacterial cell death.

Eukaryotes (e.g. animals) have 80S ribosomes which are different from the ribosomes of prokaryotes (70S). The 80S ribosomes are made up 60S larger subunit and the 40S smaller subunit. The 60S larger subunit contains 28S rRNA, 5S rRNA, 5.8 S rRNA and 47 proteins. The 40S smaller subunit has 18S rRNA and 32 proteins. Streptomycin does not bind to the 18S rRNA of 40S subunit and does not show any effect on protein synthesis in eukaryotes.