TABLE 2.
Characteristics of translational initiation sites
| Organism | AUG context and initiation site signals | AUG priority | Sensitivity to mRNA secondary structure (90% inhibition at ca.): | Position effect guiding inhibition via structure |
|---|---|---|---|---|
| E. coli | . . SD . . AUG . .a AU rich or other elementsb | AUG selection efficiency determined by local TIRd sequence and structure; coupling between AUGs in cis can be very influential | 8 kcal mol−1 in TIR | Structure restricting access to SD/AUG |
| S. cerevisiae | Preferred: AA/UAAUG . .c (context effects) | Scanning dictates a 5′-proximity selection gradient for multiple AUGs which is subject to modulation by context effects | 15 kcal mol−1e 5′ of AUG | Degree of inhibition largely independent of position of secondary structure in 5′UTR |
| Vertebrates | Preferred: C .A/GCCAUGc | Same as in yeast, but different context effects | 50 kcal mol−1 5′ of AUG | Secondary structure more inhibitory in 5′-end-proximal position |
a
SD-to-AUG distances are generally 5 to 13 nucleotides. Initiation at GUG (relative frequency, ∼8%), UUG (relative frequency, ∼1%), or AUU (one case) possible.
b
Other sequence elements may play a role in initiation, especially in the absence of a strong SD region (see summaries in references 362 and 363).
c
The significance of downstream nucleotide contexts is likely to be complex, given that these will affect the N-terminal sequence of the encoded protein. These nucleotides are therefore omitted here.
d
TIR, translational initiation region (see reference 363 for the definition used).
e
This value depends on the G+C content of the stem-loop structure (570).