RNase-sensitive DNA modification(s) initiates S. pombe mating-type switching.

Area of Science:

• Molecular Biology
• Genetics
• Cell Biology

Background:

• Mating-type switching in fission yeast is a crucial process for sexual reproduction.
• This switching is regulated by epigenetic modifications, specifically an imprint at the mat1 locus.
• Previous studies indicated this imprint is established on the lagging DNA strand during replication.

Purpose of the Study:

• To identify the molecular nature of the mat1 locus imprint.
• To elucidate the mechanism of imprint formation, including the role of replication dynamics.
• To investigate the involvement of specific proteins in regulating imprint establishment.

Main Methods:

• Biochemical analysis to characterize the imprint's composition.
• Genetic analysis using fission yeast mutants (swi1, swi3) to study replication dynamics.
• Replication timing assays to pinpoint when the imprint is formed relative to replication fork progression.

Main Results:

• The mat1 imprint is an RNase-sensitive modification, likely consisting of one or two RNA residues incorporated into the DNA.
• Imprint formation is dependent on the pausing of the replication fork, mediated by Swi1 and Swi3 proteins.
• Replication pausing and imprint establishment occur after leading-strand synthesis has passed the site and lagging-strand synthesis has initiated.

Conclusions:

• The study identifies the mat1 imprint as an RNA-containing modification essential for mating-type switching.
• A novel mechanism is proposed where lagging-strand synthesis signals Swi1/Swi3 to pause leading-strand replication, facilitating imprint formation.
• This provides insights into the coordination of DNA replication and epigenetic regulation.