INVESTIGATING GENE REGULATORY ARCHITECTURES THAT DICTATE TRANSGENERATIONAL EPIGENETIC EFFECTS IN C. ELEGANS

Abstract

The form and function of an organism rely on the recreation of similar gene expression patterns in every generation. The information for these expression patterns is stored in a single cell (e.g., zygote) in two forms – the genome sequence and the spatial arrangements of gene regulators. The interactions of regulators and the genome form intricate networks with different regulatory architectures. However, a change in the environment can impact gene expression by disrupting the physical and/or chemical properties of regulators or interactions without mutations in DNA sequence. Such epigenetic information can be transmitted across generations, but how long these effects can last is unclear. Here, we investigate the regulatory elements that promote transient or permanent epigenetic effects by analyzing the properties of a recombinant two-gene operon that expresses the fluorescent proteins mCherry and GFP and is susceptible to long-term RNA silencing in the nematode C. elegans. We reveal that 1) multiple mechanisms regulate transgenerational gene silencing and 2) the presence of the mCherry sequence can perturb RNA regulation within the germline to facilitate heritable epigenetic changes. Previous studies showed that the Argonaute protein HRDE-1 is required for the maintenance of silencing in the germline initiated by double-stranded (ds)RNAs and that poly-UG (pUG)-RNAs are key intermediates generated from the target mRNA. We found that loss of HRDE-1 can selectively rescue the expression of one cistron in a two-gene operon, suggesting that the two cistrons are not regulated by the same silencing pathway, but rather by a chromatin-independent mechanism that requires an unknown regulator. Surprisingly, we detected distinct populations of pUG-RNAs associated with expressed and silenced genes, suggesting that pUG-RNAs could potentially prime expressed genes for long-term silencing. Consistently, total RNA sequencing revealed trace amounts of anti-sense RNAs against mCherry and gfp that could trigger the production of pUG-RNAs. Examining the endogenous genes perturbed by the presence of mCherry suggests that long-term RNA silencing relies on the synergy between the sensing and processing of dsRNAs. Together, our results provide insights into the regulatory architectures and mechanisms of heritable gene silencing that occur without genetic mutations.