Question

Do the following events during bacterial translation occur primarily within the 30S subunit, within the 50S subunit, or at the interface between these two ribosomal subunits?

i) mRNA-tRNA recognition

ii) Peptidyl transfer reaction

iii) Exit of the polypeptide from the ribosome

iv) Binding of initiation factors IF1and IF3

Answer 1

(i) mRNA-tRNA recognition take place at the surface between the large and small subunit. At the interface between two ribosomal subunits translation takes place, where the mRNA template (codons) is threaded and complementary tRNA molecules (anticodons) attach.

(ii) Peptidyl transfer reaction is catalysed by the enzyme peptidyl transferase which is found in 50S subunit of ribosome. So Peptidyl transfer reaction takes place within the 50S subunit.

(iii) The polypeptide chain elongates by adding amino acids to the carboxyl end of the growing chain. This growing polypeptide exits the ribosome through the polypeptide exit tunnel in the large subunit. Hence, exit of the polypeptide occurs at 50S subunit of ribosome.

(iv) Initiation starts with the binding of IF1 and IF3 to the small subunit of ribosome. IF1 makes sure that tRNA does not bind to A site of the small ribosomal subunit and IF3 ensures that the large subunit does not bind prematurely before mRNA has bound. IF3 gets bound to the small ribosomal subunit in the previous cycle of translation itself, thus ensuring the dissociation of ribosome into its large and small subunits. So, binding of initiation factors IF1and IF3 takes place at the 30S subunit of ribosome.