X-Ray Analysis of Prokaryotic and Eukaryotic Ribosomes - Biophysical Approaches to Translational Control of Gene Expression - page 9

a b Elongation

Initiation

U1196

Gln162

Gln162

U +7

A +7

U +8

A +6

C1397

C1397

c d

Initiation Elongation

A tRNA Phe G34

C1054

C1054

U1196

U1196

Gln162

Gln162

A +9

U +9

A +8

U +8

e

Head

Body

Fig. 1.4 mRNA-ribosome interactions downstream of the A codon. ( a ) In the initiation complex
nucleotide C1397 (h28) from the neck of 16S rRNA interacts with the third nucleotide (+6) of the
A codon. ( b ) View of the interactions downstream of the A codon in the elongation complex:
nucleotide C1397 (h28) from the neck of 16S rRNA is tucked away from mRNA. ( c ) The sugar
backbone of nucleotide +9 is held in place by hydrogen bonding with Gln162 of ribosome protein
S3 in the initiation complex. ( d ) In the elongating ribosome a continuous base-stacking network
is formed between nucleotides at positions +9 and +8 which in turn stack on 16S rRNA nucle-
otides U1196 and C1054. Gln162 of protein S3 forms hydrogen bonds with mRNA nucleotide at