Question

The strain of XDR-TB that the young man had above was studied further by analyzing the DNA of the Mycobacterium tuberculosis. The investigators particularly looked at the gene for the protein targeted by the antibiotic rifampin. MDR-TB and XDR-TB are resistant to rifampin. Q1. What is the protein target for rifampin in the bacterial cells? What biological process does the antibiotic stop? (see notes). The following results were obtained from the young man's M. tuberculosis for the DNA sequence of the target gene of the rifampin antibiotic target. The normal gene sequence of the DNA coding strand is 5' AGC3' which was changed (mutation) in this young man's DNA to 5' ATC 3' Q2. What are the two mRNA codons from these two DNA triplets? No other mutations were observed in the gene. This one mutation was the reason for the resistance in the young man's Mycobacterium tuberculosis to the rifampin.

Answer 1

1. Rifampin also known as rifampicin, is an antibiotic which inhibits the DNA dependent RNA polymerase.

Target: DNA dependent RNA polymerase

Step blocked: Elongation of transcription

It binds to the $\beta$ subunit away from the active site and blocks the elongation step in transcripyion. This causes inhibition of RNA synthesis and so no protein synthesis occurs since mRNA serves as a template for protein synthesis.

RNA polymerase looks similar in shape to a crab's claw and consists of an $\alpha$ ₂$\beta$$\beta$^'$\omega$ subunits.

$\beta$$\beta$' forms the pincers of the crab's claw which holds the active site at its base. 2 metal ions are also involved in the catalytic mechanism - a tightly bound Mg²⁺ and the other Mg²⁺ is brought in along with each nucleotide being added.

Transcription involves 3 basic steps -

Initiation

Elongation

Termination

2.

The 2 mRNA codons encode for the following amino acids -

AGC = Ser

ATC = Ile

Codons are triplet nucleotide sequences which guide the process of protein synthesis. Each codon encodes for only 1 amino acids, but 1 amino acid can be encoded by >1 codons (degeneracy of codon). Codons are unambiguous and nearly universal. ATG encodes for Met and also acts as a start codon. Although in some bacterial species use GUG also as a start codon.