Specific tandem repeats are sufficient for paramutation-induced trans-generational silencing.

Area of Science:

• Epigenetics
• Plant Molecular Biology
• Gene Regulation

Background:

• Paramutation is an epigenetic phenomenon involving trans-allelic communication leading to heritable transcriptional silencing.
• Paramutation at the b1 locus in maize relies on RNA-mediated silencing and specific tandem repeats that generate small interfering RNAs (siRNAs).

Purpose of the Study:

• To determine if b1 tandem repeats are sufficient for paramutation.
• To investigate if an allelic position is necessary for paramutation mediation.
• To explore the association between paramutation ability and DNA methylation levels in repeat sequences.

Main Methods:

• Constructed multiple transgenes containing b1 tandem repeats, including partial repeat units.
• Assessed paramutation induction and reporter gene expression in maize.
• Utilized the mediator of paramutation1 gene to confirm pathway involvement.
• Analyzed siRNA production and DNA methylation levels in transgenic and endogenous loci.

Main Results:

• Tandem repeats, even partial ones, were sufficient to induce paramutation without an allelic position.
• Transgenic repeats contained regulatory sequences and required the mediator of paramutation1 gene, indicating an RNA-mediated silencing pathway.
• While all transgenes produced siRNAs, not all induced paramutation, showing siRNA production alone is insufficient.
• Transgene-induced silencing was less efficiently transmitted than endogenous paramutation; extensive DNA methylation was not required for transgene-induced paramutation.

Conclusions:

• The b1 tandem repeats are sufficient to mediate paramutation independently of their allelic position.
• Transgene-mediated paramutation utilizes an RNA-silencing pathway similar to endogenous paramutation.
• siRNA production is a prerequisite but not the sole determinant of paramutation efficacy.
• While not essential for initiation, increased DNA methylation in endogenous repeats correlates with stronger silencing after transgene-induced paramutation.