Transcription fidelity

THE FIDELITY FREQUENCY OF TRANSCRIPTION IS CONTROLLED BY PROTEINS BOUND TO CERTAIN DNA SEQUENCES... 

Boyer, J.C., Bebenek, K., Kunkel, T.A. Analyzing the fidelity of reverse transcription and transcription. Meth. Enzymol. 275, 523-537 (1996). 

The error catastrophe hypothesis suggests that through random errors in translation or transcription, erroneous copies of proteins associated with chromosomes lead to genetic abnormalities (02). This, in turn, may result in persistently abnormal protein synthesis, and an eventual error catastrophe destroys the cell. As a result, the ability of a cell to produce its normal complement of functional proteins depends not only on the correct genetic specification of the various amino acid sequences, but also on the competence and fidelity of the protein-synthesizing apparatus (i.e., the information must be translated correctly). 

In all these events, the hydrogen-bonded Watson-Crick base pair is operative and is responsible for DNA reduplication, transcription, and translation. Since mispairing can occur, all these processes are checked for fidelity by several enzymes which can correct for errors [6521. At this and all other levels of DNA reduplication and protein biosynthesis, intermolecular hydrogen bonds between nucleic acids and between nucleic acids and proteins are responsible for recognition, interaction, and, finally, for information transfer. 

RNA polymerase, like the DNA polymerases described earlier, takes instructions from a DNA template. The earliest evidence was the finding that the base composition of newly synthesized RNA is the complement of that of the DNA template strand, as exemplified by the RNA synthesized from a template of single-stranded ( ) XI74 DNA (Table 5 3). Hybridization experiments also revealed that RNA synthesized by RNA polymerase is complementary to its DNA template. In these experiments, DNA is melted and allowed to reassociate in the presence of mRNA. RNA-DNA hybrids will form if the RNA and DNA have complementary sequences. The strongest evidence for the fidelity of transcription came from base-sequence studies showing that the RNA sequence is the precise complement of the DNA template sequence (Figure 5.26). 

It is noteworthy that RNA polymerase lacks nuclease activity. Thus, in contrast with DNA polymerase, RNA polymerase does not correct the nascent polynucleotide chain. Consequently, the fidelity of transcription is much lower than that of replication. The error rate of RNA synthesis is of the order of one mistake per 10 or lO nucleotides, about 10 times as high as that of DNA synthesis. The much lower fidelity of RNA synthesis can be tolerated because mistakes are not transmitted to progeny. For most genes, many RNA transcripts are synthesized a few defective transcripts are unlikely to be harmful. 

After internalization, the virus is uncoated in preparation for replication. The genetic material of HIV is positive-sense singlestrand RNA (ssRNA) the virus must transcribe this RNA into DNA to optimally replicate in human cells (transcription normally occurs from DNA to RNA—HIV works backward, hence the name retrovirus). To do so, HIV is equipped with a unique enzyme, RNA-dependent DNA polymerase (reverse transcriptase). Reverse transcriptase first synthesizes a complementary strand of DNA using the viral RNA as a template. The RNA portion of this DNA-RNA hybrid is then partially removed by ribonuclease H (RNase H), allowing reverse transcriptase to complete the synthesis of a double-stranded DNA (dsDNA) molecule. Unfortunately, the fidelity of reverse transcriptase is poor. 

Although the replication and transcription/translation processes occur with high fidelity, occasionally mutations can occur. These can lead to death,... 

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