Bacterial Physiology (Micr430)

Lecture 8Macromolecular Synthesis and

Processing: Proteins

(Text Chapter: 10)

PROTEIN SYNTHESIS Also termed translation A triplet code specifies for an amino acid There may be more than one code for a

particular amino acid (degeneracy) Among 64 combinations of four bases,

three (UAG, UAA and UGA) do not encode any amino acid - nonsense codons

Transfer RNA tRNA possesses anticodon that

complements a specific codon on mRNA It has a specific amino acid attachment

site for binding a particular amino acid It has a ribosome recognition site It consists of about 80 nucleotides,

many of which are of unusual nucleotides

Charging of tRNA

Fig. 10.32

Composition of ribosome

Initiation of Protein Synthesis

In the absence of mRNA, 30S and 50S subunits remain separated, ensured by the activity of initiation factors 1 and 3 (IF-1 and IF-3) associated with 30S.

If mRNA is present, IF-3 binds to mRNA and thus brings mRNA and 30S together.

The Shine-Dalgarno sequence on mRNA will pair with a region located at the 3’ end of 16S rRNA

Initiation of Protein Synthesis

Next, cytosolic initiator factor 2 (IF-2) complexes with GTP and initiator tRNA (fMet-tRNA)

IF-2 directs binding of fMet-tRNA to P site within 30S subunit, permitting the association of 30S and 50S subunits

IF-3 is then removed from the complex

Initiation Complex

Formation

Fig. 10.33

Elongation of Protein Synthesis

Elongation factor Tu shuttles proper aa-tRNA into the A site, forming anticodon:codon match

The peptide bond is formed between the incoming aa and the C terminal of the elongated polypeptide, catalyzed by a ribozyme located on 50S subunit

Growing peptide bound to tRNA on A site is moved to the P site with help by Elongation factor G and GTP

Elongation Fig. 10.35

Termination When translocation brings one of the

termination codons into the A site, the ribosome will bind one of the two peptide release factors (RF-1 and RF-2)

The release factor will activate peptidyl transferase, hydrolyzing the bond joining polypeptide to the tRNA at P site.

Ribosome disassembles and recycle to another round of protein synthesis

Termination Fig. 10.35