Mechanism of R-loop formation at immunoglobulin class switch sequences

Area of Science:

• Molecular Biology
• Genetics
• Biochemistry

Background:

• R-loops, RNA-DNA hybrids, are observed in vivo but their formation mechanism is poorly understood.
• Previous studies noted R-loops at immunoglobulin class switch sequences and replication origins.

Purpose of the Study:

• To elucidate the biochemical mechanism and determinants of R-loop formation.
• To investigate the role of G-clustering in R-loop stability.

Main Methods:

• In vitro transcription assays with varying conditions.
• Addition of RNase T(1) during and after transcription.
• Transcription in the presence of Li(+) or Cs(+) ions.
• Analysis of R-loop formation efficiency with altered switch repeat sequences and G-clustering.

Main Results:

• R-loop formation is significantly reduced when RNase T(1) is added during transcription, indicating RNA must exit the polymerase.
• R-loops persist in the absence of G-quartet formation, decoupling R-loop stability from this structure.
• R-loop formation efficiency correlates with the number of switch repeats and G-clustering density.
• The G-clustering and density observed in mammalian switch regions are critical for R-loop formation.

Conclusions:

• R-loop formation is an active process involving RNA dissociation from the polymerase and competition for DNA binding.
• G-clustering in DNA sequences, particularly within switch regions, is a key determinant for R-loop formation.
• The findings suggest a primary functional role for G-clustering in facilitating R-loop formation in vivo.