Fundamentals of Molecular Biology - V.I. Rezyapkin 2009

Regulation of Gene Expression
Translation Ambiguity

For the correct reproduction of Genetic information, error-free Translation would seem to be essential. In most cases, this is indeed true. However, there are numerous Examples where genetic information is decoded in ways that deviate strictly from the rules of METABOLISM/28.html">The Genetic Code and translation. Let us examine some of them.

✵ Some mRNAs contain terminating codons within their translated region, yet these are bypassed through ribosomal frameshifting. The reading frame can shift by -1, +1, or +2.

✵ During the translation of certain genes, the ribosome is capable of making jumps spanning dozens of NUCLEOTIDES.

✵ In Retroviruses, translating the Gene that encodes Reverse Transcriptase involves a programmed one-nucleotide frameshift. Without this shift in the reading frame, the enzyme would not be produced at all.

✵ If nonsense suppression (readthrough) at the end of the gene encoding the coat protein of a certain RNA-containing bacteriophage did not occur with a frequency of 3%, its Viral Particles would fail to be infectious toward E. coli Cells.

✵ Many organisms possess tRNAs that mediate non-standard reading of the genetic code. For instance, Bacteria have been found to contain a tRNA that reads the UUU codon (phenylalanine) as a leucine codon, while Yeasts possess tRNAs that read the UAA and UAG nonsense codons as glutamine. Interestingly, their anticodons were not complementary to the codons being read.

✵ During translation, the non-standard amino acid selenocysteine can be incorporated into a protein. It is encoded by the UGA nonsense codon provided it is followed by a specific downstream stimulating sequence.

The aforementioned evidence demonstrates that the rules governing the readout of genetic information can be bypassed whenever required for optimal Gene Expression.