X-Ray Analysis of Prokaryotic and Eukaryotic Ribosomes - Biophysical Approaches to Translational Control of Gene Expression - page 14

a

tRNA

P

3 ′ mRNA

30S

S18 P

A

3 ′ mRNA

5

II

30S

III

S2

5 ′ mRNA

3 ′ mRNA

30S

5 ′ mRNA

P

A

E

S2

I

30S

IV

3 ′ mRNA

5 ′ mRNA

Fig. 1.7 Simpli fi ed scheme for messenger RNA motion on the ribosome

single-stranded 3¢ end of 16S rRNA. Subsequently, the SD duplex interacts with the
30S platform and is oriented towards ribosomal protein S2, while the rest of the
mRNA molecule is still unbound (Fig. 1.7 , state I). The following step is a simulta-
neous positional adjustment of the initiation codon in the P site, where it is stabi-
lized by a codon-anticodon interaction with initiator tRNA fMet , and a rearrangement
of the SD duplex towards ribosomal protein S18 (Fig. 1.7 , state II). At this stage of
initiation, the mRNA is present in a tense conformation and is precisely positioned
on the 30S subunit, so that the start codon will be read first and in the correct frame.
Our findings of mRNA adjustments during the initiation process are in good agree-
ment with earlier cross-linking results (Canonaco et al. 1989 ; Rinke-Appel et al.
1994 ; La Teana et al. 1995 ). Immediately after initiation, when one or several codons
have already been translated by the ribosome but the SD interaction is still intact, a
simultaneous movement of the complete mRNA in the 5¢ end direction and a length-
ening of the SD helix take place (Fig. 1.7 , state III). The SD helix is once again
shifted towards ribosomal protein S2. Further translation of mRNA by the ribosome
is accompanied by movement in the 3¢ -5 ¢ direction and leads to melting of the SD
interaction. Eventually, at some stage during elongation, the 5¢ end of mRNA will
no longer interact with the ribosome at all (Fig. 1.7 , state IV) and become accessible
for other ribosomes, which can lead to formation of a polysome.