During protein biosynthesis, readthrough in translation is a nonstandard decoding of the stop codon to an amino acid, allowing the ribosome to continue translation beyond the termination codon (1). This phenomenon is also called nonsense suppression. Readthrough is caused by either mutations in the translation machinery or by specific signals on the messenger RNA. Genetically programmed readthrough in response to the mRNA signals is included in the recoding that represents reprogrammed genetic decoding [see Frameshifting (2)]. In a broader sense, translation beyond the stop codon by any mechanism, including frameshifting, is sometimes referred as readthrough. The term readthrough is also used for transcription beyond the transcript terminator.
Nonsense suppressor tRNAs are mutant transfer RNAs that decode one or more termination codons, usually due to a point mutation in the anticodon. Some of the naturally occurring tRNAs show similar readthrough activity at the stop codon and are referred to as natural suppressor tRNAs. Certain mutations of ribosomes or release factors exhibit various nonsense suppressor activities in the absence of a suppressor tRNA. These mutations enhance the frequency of stop-codon misreading with a natural tRNA by affecting the normal termination process.
The programmed readthrough is further divided into two classes. In one class, canonical amino acids are inserted for the termination codons. At least two cellular genes utilize this type of readthrough. The gene for E. coli pili assembly is expressed by readthrough of a UAG codon. Drosophila kelch mRNA uses UGA readthrough. A number of virus genes have termination codons to be readthrough. A C-type retrovirus, murine leukemia virus (MuLV), has a UAG codon that is decoded as a glutamine at residue an efficiency of 5% to express the gag-pol polyprotein. An animal virus, sindbis, reads a terminating UGA as tryptophan at 10% efficiency. Various RNA plant viruses and bacteriophages also use UGA or UAG readthrough. These phenomena are mRNA- and codon-specific, involving specific signals on the mRNAs. In the case of MuLV, the UAG codon is accompanied by a downstream pseudoknot (see RNA Structure) structure that stimulates the readthrough.
The other category of programmed readthrough is insertion of a selenocysteine residue at certain UGA codons to synthesize selenoproteins (3). Selenoproteins exist in many organisms of all the three superkingdoms—archea, eubacteria, and eukarya—except for some yeast species. In higher eukaryotes, nearly 10 selenoproteins have been identified. The UGA codons in the mRNA are decoded by a specific selenocysteinyl-tRNA encoded by a chromosomal gene. Selenocysteine tRNAs have a unique secondary structure. The tRNA is first aminoacylated with serine, which is subsequently converted to selenocysteine on the tRNA by specific enzymes. Stem loop structures of the mRNA, termed selenocysteine insertion sequence (SECIS), serves as a signal for the insertion. The SECIS is located just 3′ to the UGA codon in bacterial selenoprotein mRNA, while it is on the 3′ untranslated region in animal selenoprotein mRNAs. In bacteria, a special elongation factor, SelB, instead of EF-Tu, delivers selenocysteyl-tRNA to the A site of the ribosome (Fig. 1). The precise mechanism of selenocysteine insertion in animals is not known.
Figure 1. Schematic representation of decoding of the UGA codon to selenocysteine (Sec) on the ribosome using the specific tRNA, elongation factor SELB, and the SECIS signal.
