G-quadruplex structures in RNA stimulate mitochondrial transcription termination and primer formation

Area of Science:

• Molecular Biology
• Genetics
• Biochemistry

Background:

• The human mitochondrial transcription machinery produces primers essential for initiating leading-strand DNA replication.
• A prevailing model suggests primer length is determined by transcription termination at conserved sequence block II (CSB II) within the mitochondrial DNA control region.

Purpose of the Study:

• To investigate the molecular mechanism underlying transcription termination at CSB II in human mitochondria.
• To elucidate the role of RNA structures in defining the 3' end of mitochondrial DNA replication primers.

Main Methods:

• Analysis of RNA secondary structures formed during transcription of the mitochondrial DNA control region.
• Experimental investigation of transcription termination events at CSB II.
• Assessment of the influence of downstream DNA sequences on termination efficiency.

Main Results:

• Demonstration that G-quadruplex structures forming in nascent RNA upon transcription of CSB II are responsible for site-specific transcription termination.
• Evidence that a downstream poly-dT stretch modestly enhances termination efficiency at CSB II.
• Comparison of the observed termination mechanism to Rho-independent transcription termination in prokaryotes, highlighting the unique role of G-quadruplexes.

Conclusions:

• G-quadruplex formation in RNA at CSB II is the key determinant of transcription termination in human mitochondria.
• This RNA-based termination mechanism dictates the precise length of primers for mitochondrial DNA replication.
• The findings reveal a novel RNA structure-dependent transcription termination strategy in eukaryotic mitochondria.