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Lake Model

In Lake’s model the 30S subunit is considered to be completely asymmetrical. An indentation divides the subunit into two unequal parts, referred to as the upper one-third (head) and the lower two-third (body). Extending from the lower two-third is a region called the platform. There is a cleft between the platform and the upper one-third. This cleft is an important functional region. It has been proposed as the site of the cod on – anticodon interaction and as a part of the binding site for initiation factors IF – 1, IF – 2 and IF – 3. The 30S subunit is Lake’s model differs from that of Stoffler and Wittmann’s model is that it is completely asymmetrical and does not contain a mirror plane. This results in a marked difference in protein location, with completely different functional implications. The cleft between the head and the body found in Stoffler and Wittmann’s model is not evident in Lake’s model.

lake model

Functional sites of the ribosome. Lake has given an account of the functional sites of the E.coll 70S ribosome.

(1) Initiation factor 3 (IF-3), which is required for binding natural mRNA to the small subunit, binds to a site near the cleft. The site includes a part of the platform and a part of the one-third region.

(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 interaction.

(4) Protein S5 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 is located on the external side, and the IF – 3 binding site is on the internal side, a tRNA molecule cannot simultaneously be in contact with mRNA and peptidyl transferase and still contact S5.

(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-L12. Elongation factors EF-Tu and EF-G are associated with these proteins.

(9) The 5S RNA site is indicated by protein L5 and L25.

(10) L11 is suggested to be near the peptidyl tranferase P site.

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