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Index >> Bacterial Structure >> Functional Sites of Ribosomes

Functional Sites of Ribosomes

	Functional Sites of Ribosomes - Lake (1978) has given an account of the functional sites of the E.coli 70S ribosome. (I) Initiation factor 3 (IF-3), which is required for binding natural mRNA to the small subunit, binds to a site near the cleft. This site includes a part of the platform and a part of the one-third region. IF-linkstoproteins Sll, S12, S13, SI9 and S71ocatedinthis site. The cleft has also been proposed to be the binding site for1F-l and IF-2 (initiation factors). (2) The P site is also suggested to be located in the region near the cleft.
(3) Because proteins S1, S4 and S11 have been mapped in the cleft, this region is surmised to be the site of codon-anticodon inter action. (4) Protein SS is implicated in tRNA binding and codon recognition, and is located on the exterior (cytoplasmic side) of the small subunit. (5) Transfer RNAs of' both A and P sites are located on the 30S- 50S interface. (6) Since protein S5(tRNA binding and codon recognition) is located on the external side, and the IF-3 binding site is (in the internal (interface) side, a tRNA molecule cannot simultaneously be in contact with mRNA and peptidyl transferase and still contact S5.
To explain this Lake (l97) developed a new concept, the recognition site (R). S5 is likely to be part of the R site. Aminoacyl tRNA is presumed to first bind to the R site and then switch over to the A site. This change is postulated to involve a conformation change in the anticodon loop of the tRNA molecule. The R site concept is described as a functionally and physically distinct state of aa-tRNA during initial codon recognition. (7) EF-Tu is probably near the cluster S19. S10-I and S4-I.(8) The elongated left projection of the large subunit contains proteins L7-LI2. Elongation factors EF-Tu and EF-G is associated with these proteins.
(In Stoffler and Wittmann's model the proteins L7/L12 occur in three to four copies, forming a garland on the central protuberance). (9) The 5S RNA site is in1icated by proteins L5 and L25. (10) L 11 is suggested to be near the/ peptidyl transferase P site

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