Page 108 - Academic Press Encyclopedia of Physical Science and Technology 3rd Molecular Biology

P1: GTQ Final pages
Encyclopedia of Physical Science and Technology EN017F-788 August 3, 2001 16:27

Translation of RNA to Protein 49

D. Control by Modification of Translation senger RNA translation. In prokaryotes, the most effective
Factor Activity inhibitors appear to have a base sequence complementary

to the 5 leader region, including the Shine–Dalgarno se-
1. Initiation Factors
quence, which is involved in the binding of mRNA to
In eukaryotes, initiation of protein synthesis is inhibited the small ribosomal subunit. In eukaryotes, translation of
by phosphorylation of the initiation factor eIF-2. Phos- mRNA is inhibited by polyribonucleotides complemen-

phorylation is stimulated by the lack of haem or the tary to the 5 untranslated region of mRNA, indicating a di-
presence of double-stranded RNA. Two different pro- rect effect on initiation as in prokaryotes. Polynucleotides

tein kinases capable of phosphorylating the alpha sub- complementary to the 3 untranslated region of mRNA
unit of eIF-2 have been characterized. One enzyme, called also inhibit translation in some cells, and this effect may
the haem-controlled repressor (HCR) or haem-regulated be due to destabilization of mRNA by ribonucleases spe-
inhibitor (HRI), is a cytoplasmic protein (95,000 Da) that cific for double-stranded RNA. The effect of antisense
becomes activated by phosphorylation. The other kinase polynucleotides is not restricted to translation of mRNAs,
(67,000 Da) is activated by phosphorylation in the pres- but transcription as well as the processing of transcripts
ence of double-stranded RNA. Thus, a cascade of pro- may also be inhibited.
tein phosphorylation is involved. Phosphorylated eIF-2
is unable to exchange GDP for GTP, which prevents F. Availability of Amino Acids, tRNA
it from functioning in the binding of initiator tRNA to Abundance, and Codon Usage
ribosomes.
Conditions other than lack of haem (e.g., heat shock, 1. Amino Acids
serum deprivation, or the presence of oxidized glu- Polypeptide synthesis depends on an adequate supply
tathione), which are known to inhibit protein synthesis, of tRNAs charged with the 20 protein amino acids and
also give rise to the phosphorylation of eIF-2α. Con- appropriate interactions between their anticodons and the
versely, the activity of eIF-4F is decreased by dephospho- codons of mRNA. Peptide chain elongation is decreased or
rylation of the 24-Da subunit (see Table III) rather than by inhibited by lack of amino acids or other conditions giving
phosphorylation. Thus, a number of different kinases and rise to an imbalance or deficiency in aminoacyl-tRNAs.
phosphatases are involved in modulating the activities of
different factors. 2. Abundance of tRNAs and Codon Usage
Small RNAs may also be involved in regulating the
translation of mRNA in eukaryotic cells. Of the stimula- Different tRNAs are present in the cytosol in unequal
tory RNAs, the best characterized is a small RNA of about amounts, and elongation rates are slower at codons corre-
160 nucleotides, which accumulates in cells after infection sponding to rare tRNA species.
with adenovirus. This virus-associated RNA, VA-RNA 1 , The existence of synonymous codons raises the ques-
which is required to maintain general protein synthesis, tion of preferential use of some codons and its possible sig-
acts by inhibiting the phosphorylation of the alpha sub- nificance in relation to translational efficiency and control.
unit of initiation factor eIF-2. In some bacteria (e.g., Pseudomonas aeruginosa, which
has a high content of G + C, 67.2%, in DNA), the most
common codons are those with the strongest predicted
2. Other Translation Factors
codon–anticodon interaction—that is, G + C base pairs—
2+
A Ca /calmodulin-dependent protein kinase phospho- but this preference is not universal and, for example, does
rylates eEF-2. The phosphorylated factor appears to be not apply to E. coli, which has a lower proportion of G + C
inactive and moreover also inhibits the activity of the non- (50%). Although codon usage may play a part in deter-
phosphorylated factor. Dephosphorylation of the factor by mining elongation rates, it is probably of less importance
phosphatase restores its activity. in translational control than the secondary structure of
mRNA in relation to the rate of initiation of protein syn-
thesis.
E. Effects of Antisense Polynucleotides
Antisense RNAs, which are polynucleotides with base se-
G. Modulation of Ribosome Activity
quences complementary to messenger RNAs, have been
found in both prokaryotes and eukaryotes. Natural anti- Specific ribosomal components have an important func-
sense RNAs are not common but synthetic RNAs directed tion in relation to the fidelity of protein synthesis. Thus,
at specific targets have been widely studied. It has been in E. coli ribosomal protein S12 determines the accu-
demonstrated that they can function as inhibitors of mes- racy of codon–anticodon interactions and modulates the