Termination signal

In molecular biology, a termination signal is a sequence that signals the end of transcription or translation.^[1] Termination signals are found at the end of the part of the chromosome being transcribed during transcription of mRNA. Termination signals bring a stop to transcription, ensuring that only gene-encoding parts of the chromosome are transcribed.^[1] Transcription begins at the promoter when RNA polymerase, an enzyme that facilitates transcription of DNA into mRNA, binds to a promoter, unwinds the helical structure of the DNA, and uses the single-stranded DNA as a template to synthesize RNA.^[1] Once RNA polymerase reaches the termination signal, transcription is terminated.^[1] In bacteria, there are two main types of termination signals: intrinsic and factor-dependent terminators.^[1] In the context of translation, a termination signal is the stop codon on the mRNA that elicits the release of the growing peptide from the ribosome.^[2]

Overview of transcription process. Termination of transcription occurs due to termination signal.

Termination signals play an important role in regulating gene expression since they mark the end of a gene transcript and determine which DNA sequences are expressed in the cell.^[1] Expression levels of certain genes can be increased by inhibiting signal terminators, known as antitermination, which allows for transcription to continue beyond the termination signal site.^[1] This can be desirable under specific cell conditions.^[1]

Additionally, sometimes, termination signals are overlooked in transcription and translation, resulting in unwanted transcription or translation past the termination signal.^[3] To address this issue, termination signals can be optimized to increase termination efficiency.^[3]

Bacterial Termination Signals

Intrinsic terminator containing an RNA hairpin rich in guanine and cytosine as well as a region high in uracil

The two types of termination signals in bacteria are intrinsic and factor-dependent terminators.^[4] Intrinsic termination occurs when a specific sequence on the growing RNA strand elicits detachment of RNA polymerase from the RNA-DNA complex.^[4] In E. coli, one intrinsic termination signal consists of an RNA hairpin that has high amounts of guanine and cytosine, as well as a region high in uracil nucleobases.^[4]

Factor-dependent terminators require proteins for proper termination.^[4] One example is rho-dependent termination, a common termination mechanism found in bacteria that involves the binding of Rho protein to remove RNA polymerase from the DNA-RNA complex.^[4]

Antitermination

Antitermination involves the inhibition of signal terminators.^[4] RNA polymerase is prevented from detaching from the RNA in response to a termination signal, increasing downstream gene expression.^[4]

Antitermination can occur in a variety of ways.^[4] Some antiterminators disrupt termination signals by inhibiting RNA hairpin generation, while other antiterminators are proteins that bind to RNA polymerase and cause RNA polymerase to continue transcription past termination signals.^[4] Depending on the environment of the cell, antitermination may be crucial to cell survival.^[4] These antitermination mechanisms are crucial when the cell is under stress, allowing for increased expression of downstream genes that are needed under dire circumstances.^[4]

Termination Signal Efficiency

Transcription

Termination efficiency of T7 RNA polymerase is around 74%, which creates issues when T7 RNA polymerase is used to produce recombinant proteins.^[3] In this process, the target gene is inserted into a plasmid and is regulated by the T7 promoter; T7 RNA polymerase is used to transcribe the target gene.^[3] Due to termination inefficiency, read-through can result in increased regulation of downstream genes that may be crucial to host cell function.^[3] Selectable marker genes that are downstream of the target gene insertion site and genes that encode regulatory proteins may have altered expression as a result.^[3] Hence, proper termination of transcription is needed for plasmid stability in host cells for the proper production of recombinant proteins.^[3] Research has been conducted to identify termination signals that yield higher termination efficiency by engineering termination signals from a variety of termination signal components.^[3] Some engineered termination signals have yielded a termination efficiency as high as 99%, which is a significant improvement from the native termination efficiency associated with T7 RNA polymerase of 74%.^[3]

Translation

In translation, termination efficiency is dependent on the context of the termination signal (stop codon).^[2] Traditionally, the termination signal for translation is a 3 nucleobase sequence called a stop codon.^[2] Research has shown that the nucleobases surrounding the stop codon can impact termination efficiency.^[2] Specifically, the 4th base (nucleobase directly following the stop codon) has a significant impact on the termination efficiency.^[2] In particular, when the nucleobase at the 4th position is a purine (adenine or guanine), termination efficiency is improved.^[2] Pyrimidines (cytosine or uracil) in the 4th position result in lower termination efficiency.^[2] It has been found that highly expressed genes have higher termination efficiency due to the presence of a purine in the 4th position.^[2]