I3 - DNA ,RNA as drug targets ,replication ,transcription and translation 2

Why is RNA important as an antibacterial drug target?

• RNA polymerase is essential for bacterial transcription (DNA → RNA)

• It synthesises RNA using DNA as a template

• Bacterial RNA polymerase differs structurally from human RNA polymerases

• These differences allow selective targeting by antibacterial drugs

• Rifampicin - an antibiotic that targets bacterial RNA polymerase

How does rifampicin work?

● Rifampicin is an antibiotic that targets bacterial RNA polymerase
● It binds to the β subunit* of bacterial RNA polymerase
● This prevents the extension of newly initiated RNA template strands
● As a result, bacterial RNA synthesis is inhibited
● Without RNA – no protein synthesis growth/survival

What is bacterial translation?

● Translation is the synthesis of proteins using mRNA as a template
● Takes place on ribosomes in the bacterial cytoplasm
● The mRNA sequence determines the order of amino acids
● tRNA (transfer RNA)delivers amino acids to the ribosome
● Amino acids are joined to form a polypeptide chain, which folds into a protein

Note - Prokaryotic translation can start while transcription is still ongoing!

What makes up the genetic code - codons and mrna ?

● mRNA is read in groups of three bases called codons
● Each codon specifies an amino acid or a stop signal
● AUG is the usual start codon (methionine)
● There are 64 codons, including three stop codons
● The code is degenerate – several codons can specify the same amino acid
● Codon sequence determines the amino acid sequence of a protein

What is tRNA and anticodons?

● tRNA carries amino acids to the ribosome
● Each tRNA has an anticodon complementary to an mRNA codon
● Aminoacyl-tRNA synthetases attach the correct amino acid to each tRNA
● Codon-anticodon pairing ensures the correct amino acid is added to the growing polypeptide

Describe the bacterial ribosomes

Bacterial ribosomes are 70S, consisting of two subunits:
● The 30S subunit binds mRNA and helps decode codons
● The 50S subunit catalyses peptide bond formation

Human cytoplasmic ribosomes are 80S
(40S + 60S)

● These structural differences allow selective antibacterial drug targeting

How is bacterial translation initiated ?

● The 30S ribosomal subunit binds to mRNA near the start codon (AUG)
● The initiator tRNA carries formyl methionine (fMet)
● The initiator tRNA binds to the P site of the ribosome
● The 50S subunit joins, forming the functional 70S ribosome
● Translation is ready to begin

What is the elongation phase in translation ( sites) ?

● A site (Aminoacyl): Incoming tRNA delivers the next amino acid
● P site (Peptidyl): Holds the tRNA carrying the growing polypeptide
● Peptide bonds form between amino acids
● E site (Exit): Empty tRNA leaves the ribosome
● The ribosome moves along mRNA 5′ → 3′, repeating the cycle

How is translation terminated?

● Translation ends when a stop codon (UAA, UAG or UGA) enters the A site
● Stop codons do not code for amino acids
● Release factors recognise the stop codon and trigger release of the polypeptide
● The ribosome separates into its subunits, ready for reuse

Describe the process of translation (protein synthesis) in bacteria.

1. Initiation

• The 30S ribosomal subunit binds to mRNA near the start codon AUG.

• An initiator tRNA carrying formylmethionine (fMet) binds to the start codon at the ribosome's P site.

• The 50S ribosomal subunit joins the 30S subunit, forming the functional 70S ribosome.

• Translation is now ready to begin.

2. Elongation

• The ribosome moves along the mRNA in the 5′ → 3′ direction, reading three-base sequences called codons.

• tRNA molecules carry specific amino acids to the ribosome, with their anticodons pairing with complementary mRNA codons.

• The ribosome has three sites:

  • A site (Aminoacyl): Incoming tRNA delivers the next amino acid.

  • P site (Peptidyl): Holds the tRNA carrying the growing polypeptide chain.

  • E site (Exit): Empty tRNA leaves the ribosome.

• The 50S subunit catalyses peptide bond formation, joining amino acids together.

• The ribosome moves forward by one codon, and the cycle repeats, extending the polypeptide chain.

3. Termination

• Translation ends when a stop codon (UAA, UAG or UGA) enters the A site.

• Stop codons do not code for amino acids, so release factors bind instead of tRNA.

• Release factors trigger the release of the completed polypeptide chain.

• The ribosome separates into its 30S and 50S subunits, which can be reused.

• The polypeptide folds to form a functional protein.

How are bacterial ribosomes antibiotic targets?

● Bacterial ribosomes are important targets for antibiotics
● Antibiotics can bind to either the 30S or 50S subunit
● Tetracyclines (30S): Prevent aminoacyl-tRNA binding to the A site
● Macrolides (50S): Block the peptide exit tunnel, disrupting elongation

summary of antibiotic targets