DNA-dependent protein kinase specifically represses promoter-directed transcription initiation by RNA polymerase I

Area of Science:

• Molecular Biology
• Biochemistry
• Gene Regulation

Background:

• DNA-dependent protein kinase (DNA-PK) is a nuclear enzyme involved in phosphorylating transcription factors.
• The precise cellular function of DNA-PK, particularly in transcription, remains largely unelucidated.
• Ribosomal gene transcription is a critical process for cell growth and proliferation.

Purpose of the Study:

• To investigate the role of DNA-PK in regulating gene transcription.
• To determine the specific RNA polymerase and template type affected by DNA-PK activity.
• To identify the molecular mechanism underlying DNA-PK's effect on transcription.

Main Methods:

• In vitro transcription assays using Xenopus RNA polymerases I, II, and III.
• Employing linear and circular DNA templates.
• Utilizing 6-dimethylaminopurine, a protein kinase inhibitor.
• Fractionation of transcription system components to identify mediating factors.

Main Results:

• DNA-PK significantly inhibits promoter-directed transcription initiation by Xenopus RNA polymerase I in vitro.
• The inhibitory effect is dependent on protein phosphorylation and is reversed by 6-dimethylaminopurine.
• DNA-PK exhibits greater repression on linear DNA templates compared to circular ones.
• DNA-PK does not affect transcription mediated by RNA polymerases II and III.
• A protein fraction containing transcription factor Rib1 (Xenopus SL1 homolog) mediates DNA-PK's repressive effect.

Conclusions:

• DNA-PK plays a role in down-regulating ribosomal gene transcription.
• The kinase activity of DNA-PK is crucial for its inhibitory function.
• DNA-PK's impact on transcription is specific to RNA polymerase I and influenced by DNA topology.