Regulation at the Level of Translation

In order to allow a better rmderstanding of regulation at the level of translation, some of the specific features of eucaryotic translation will be summarized briefly. 

As opposed to the case in procaryotes, eucaryotic translation does not require a specific sequence for the binding of the ribosome. Procaryotes rely on the Shine-Dalgarno sequence, which is complementary to sequences of the 16S RNA of the 308 subunit. The Shine-Dalgamo sequence mediates the binding of mRNA to the 308 ribosome and ensures the correct positioning of the AUG initiation codon. 

The mRNA of eucaryotes does not possess specific initiation sequences. Rather, the AUG start codon is identified by scanning the eucaryotic mRNA the 408 subunit of the ribosome threads the 5 non-translated end of the mRNA and uses the first AUG codon encoimtered to initiate translation. Whether a AUG codon is used as an initiator depends, additionally, upon the sequence context. If the sequence environment is unfavorable for initiation, then the scanning is continued and initiation occurs at one of the next AUG. With the help of this leaky scanning strategy it is possible to produce proteins with different N-termini from the same mRNA. 8ince there are often signal sequences foimd at the N-terminus, this mechanism may lead to alternative com-partmentalization of a protein. 

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Regulation at the level of translation assumes a much more prominent role in eukaryotes than in bacteria and is observed in a range of cellular situations. In contrast to the tight coupling of transcription and translation in bacteria, the transcripts generated in a eukaryotic nucleus... 

The synthesis of ribosomal proteins is regulated at the level of translation. Certain ribosomal proteins bind to specific sites on the ribosomal RNAs or their own mRNAs. In the absence of the ribosomal RNAs, they bind to their own mRNAs, which inhibits their translation. This form of translational control regulates the rate of synthesis of ribosomal proteins so that it does not exceed the rate of ribosomal RNA synthesis. 

Alternative Modes of mRNA Splicing Present a Potent Mechanism for Posttranscriptional Regulation Gene Expression Is Also Regulated at the Levels of Translation and Polypeptide Processing Patterns of Regulation Associated with Developmental Processes... 

Gene Expression Is Also Regulated at the Levels of Translation and Polypeptide Processing... 

Gene expression can also be regulated at the level of translation. In prokaryotes, many operons important in amino acid biosynthesis are regulated by attenuation, a process that depends on the formation of alternative structures in mRNA, one of which favors transcriptional termination. Attenuation is mediated by the translation of a leader region of mRNA. A ribosome stalled by the absence of an aminoacyl-tRNA needed to translate the leader mRNA alters the structure of mRNA so that RNA polymerase transcribes the operon beyond the attenuator site. 

Synthesis of some proteins is regulated at the level of translation. 

Day and Shah [43], Shah and Day [44], and Yip et al. [42] showed that some constitutive strains of B. cereus (569/H, 5, and 5/B) responded to the additions of penicillins and cephalosporins by altering their rate of penicillinase synthesis, sometimes less than the control level and more often above the control rate twice the control rate was the greatest effect observed. It is possible that some vestige of response to the inducer still remains in these strains. Day and his co-workers suggest that regulation at the level of translation is involved, but the evidence does not appear to be conclusive. 

Gene expression is also regulated at other levels, such as nuclear to cytoplasmic transport of mRNA, translational eflticiency of mRNA, RNA and protein stability, or protein modification. As is the case for transcriptional regulation, control of gene expression at the level of translation often occurs at the initiation step of the decoding process. Thus, not all mRNAs that reach the cytoplasm are used directly to synthesize protein. In fact, as much as 10% of genes in a eukaryotic cell may be regulated at the level of translation. 

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