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Distinct modes of transcription read through or terminate at the c-myc attenuator
1Imperial Cancer Research Fund, Lincoln's Inn Fields, London, UK.
Abstract:
Premature termination of transcription by RNA polymerase II (pol II) occurs in the 5' region of many viral and cellular genes. Modulation of this process, or attenuation, is an important means of transcriptional control, but its mechanism is unknown. Using injected Xenopus oocytes, the efficiency of the mouse c-myc attenuator was tested when it was placed at various distances from the transcription initiation site. The attenuator functioned with each of six different pol II promoters tested; however, termination efficiency declined markedly when it was placed more than approximately 400 bases from the start site. This decline in attenuator function with distance from the start site coincided with increased sensitivity to the pol II inhibitor 5,6-dichloro-1-beta-D-ribofuranosyl benzimidazole (DRB). Thus transcription complexes situated further from the promoter appear to have a lower ability to recognize the attenuator and a greater sensitivity to DRB. Furthermore, polymerases which have read through one attenuation site have a reduced ability to terminate at a second site. The results imply that a discrete subset of elongation complexes is capable of premature termination, and that this subset exists only within the first few hundred bases of the transcription unit. Regulation of termination efficiency may be effected by changing the balance between the two modes of transcription committed either to read through or to terminate prematurely.
Insights
Premature transcription termination (attenuation) is distance-dependent. RNA polymerase II (pol II) complexes lose attenuation ability and gain DRB sensitivity beyond 400 bases from the start site.
Area of Science:
- Molecular Biology
- Gene Regulation
- Transcriptional Control
Background:
- Premature transcription termination, or attenuation, is a key mechanism for controlling gene expression.
- The precise mechanism and regulatory factors governing attenuation remain largely unknown.
Purpose of the Study:
- To investigate the distance-dependent efficiency of the mouse c-myc attenuator.
- To explore the relationship between attenuator function, distance from the transcription start site, and RNA polymerase II (pol II) inhibitor sensitivity.
Main Methods:
- Utilized injected Xenopus oocytes to test the mouse c-myc attenuator's efficiency.
- Assessed attenuator function across various distances from six different pol II promoters.
- Examined the impact of the pol II inhibitor 5,6-dichloro-1-beta-D-ribofuranosyl benzimidazole (DRB) on transcription complexes.
Main Results:
- The c-myc attenuator functioned effectively up to approximately 400 bases from the transcription start site.
- Termination efficiency significantly decreased beyond this distance, coinciding with increased DRB sensitivity.
- Transcription complexes further from the promoter showed reduced attenuator recognition and heightened DRB sensitivity.
- Polymerases that bypassed an initial attenuation site had diminished capacity for subsequent termination.
Conclusions:
- A specific subset of transcription elongation complexes is responsible for premature termination.
- This attenuating capability is restricted to the initial few hundred bases of a transcription unit.
- Transcriptional regulation may involve modulating the balance between read-through and premature termination modes.