Microbial Genetics (Micr340)

Lecture 4Gene Expression: Translation

Proteins

Protein structure

Its basic monomer (“link” of a chain): amino acid (a.a)

Amino acids are linked by peptide bonds, forming polypeptides (a short chain of amino acids - oligopeptide).

Peptide chains have direction (orientation) too; N-terminus (amino terminus with an unattached amino group while C terminus has an unattached carboxyl group

Peptide bond formation

Protein structure

Primary structure: sequence of a.a. Secondary structure: parts of peptide

chain are held together by H-bonds – helices and -sheets.

Tertiary structure: various regions of peptide folds up on itself; hydrophobic a.a. inside, hydrophilic a.a. outside

Quaternary structure: proteins are made up by multiple polypeptides -multimeric

Translation

Translation (protein synthesis)

Occurs on ribosomes Ribosome is huge:

3 different rRNAs over 50 proteins

Composition of ribosome

Translation (protein synthesis)

Open reading frame (ORF) Three nucleotides form a codon One codon encodes a specific amino

acid For a given DNA fragment, there are

three different ways (or reading frames) that nucleotide sequences translated into protein sequences

Aminoacylation of tRNA

Translation initiation

Translational initiation regions (TIRs) All TIRs have an initiation codon; usually

AUG or GUG Initiation codons encode methionine Many genes have Shine-Delgarno

sequence (S-D) 5 to 10 nucleotides upstream of initiation codon.

S-D sequence usually A, G rich

Translation Initiation

Translation Initiation

Translation elongation

During translation, ribosome moves 3 nucleotides at a time along mRNA

This leaves space open for another tRNA to enter; which type depends on the anticodon matching the next codon on mRNA

EF-Tu helps the correct tRNA enter the A (acceptor) site on the ribosome

Translation elongation

Ribozyme peptidyltransferase catalyzes formation of a peptide bond between the incoming a.a. at the A site and the growing polypeptide at an adjacent site (P site)

EF-G (translocase) enters ribosome and moves polypeptide-containing tRNA to P site

The tRNA which has been displaced then moves to yet another site, E site before it exits the ribosome

Overview of translation

Overview of translation

Translation termination

When ribosome moves to one of 3 stop codons (UAA, UAG and UGA), translation stops

The stop codons do not encode any a.a. so they have no corresponding tRNA; they are called nonsense codons

Termination requires release factors

Translation termination

Polycistronic mRNA