Using silencer-seq, the Stark lab identified 800+ silencers and the transcription factor Saft in fruit flies, enhancing our understanding of gene regulation.
The precise regulation of gene activity is crucial for cell specialization and the development of healthy organisms. While significant progress has been made in understanding enhancers—DNA sequences that activate genes—silencers, which suppress gene activity, remain far less studied despite their equally vital role. Technical limitations and research biases have historically made it challenging to identify silencers across the genome. However, Alexander Stark’s lab at the IMP has overcome these obstacles with a new technique called “Silencer-seq.” Using this method, they uncovered hundreds of silencers in the fruit fly genome, a breakthrough now detailed in the journal Molecular Cell.
Gene Regulation and Its Role in Cell Specialization
The development of a healthy organism depends on the precise regulation of gene activity—determining when specific genes are switched on or off. This process, called gene regulation, is critical for cell specialization, guiding cells to develop into distinct types like muscle, nerve, or skin cells, which together form a functioning organism.
For over 40 years, scientists have recognized the importance of two types of regulatory DNA sequences located away from genes: enhancers and silencers. Enhancers act as activators, “switching on” genes and increasing their expression when needed. In contrast, silencers act as repressors, “switching off” specific genes to maintain balance and ensure proper cellular function.
Research on enhancers increased the understanding of enhancers in recent years, with the Stark lab making remarkable strides, from reading their DNA sequences and understanding their gene-regulatory functions to designing tissue-specific enhancers from scratch. However, despite their critical role, scientists still know only a few silencers and don’t understand how they look and function as much.
Challenges in Identifying Silencers
The challenge mostly lies in scouting for silencers across the entirety of the genome. “Over the past decade, researchers have focused on mapping sequences that look like enhancers, assuming silencers are just enhancers in reverse,” says Lorena Hofbauer, first author of the study and a recent graduate of the Vienna BioCenter PhD Program. “They’re thought to share similar features, such as being located in open chromatin, but shutting genes down instead of switching them on.”
While several tools were developed to study elements with enhancer-like features, this focus has created a bias. As a result, tools specifically designed to detect silencers based on their gene-repressive function lag behind, leaving these genetic sequences overlooked and poorly understood.
A Pioneering Approach: The Silencer-Seq Method
Researchers from the Stark lab have now taken a pioneering, unbiased approach to find silencers in the genome of the fruit fly Drosophila melanogaster. Using ‘silencer-seq’, a new method developed for this task, the scientists discovered hundreds of silencers that differ from traditional enhancer profiles, revealing previously hidden regulatory elements in the genome. Their findings are now published in the journal Molecular Cell.
To address this gap, the Stark lab created “Silencer-seq,” a novel method based on the STARR-seq technology they previously developed.
Revealing Silencer Mechanisms With Silencer-Seq
The approach begins by creating a comprehensive library of DNA fragments, each representing a small segment of the fruit fly genome. These fragments, which together represent the entire genome, are paired with a strong enhancer designed to drive transcription—unless the DNA fragment acts as a silencer, actively repressing the process.
The resulting DNA constructs are introduced into fruit fly cells, where they interact with the cells’ transcription machinery. Each fragment’s activity is assessed by monitoring messenger RNA (mRNA) output: fragments that suppress transcription of the reporter gene lead to little or no mRNA production, identifying them as silencers. By sequencing the mRNA, researchers can then pinpoint which genomic fragments possess silencing activity.
“We found over 800 silencers across the entire fruit fly genome with this tool,” says Hofbauer “This is the first time an unbiased large-scale catalog of these regulatory elements is assembled.”
With access to such a collection of silencers, the team was able to identify key features shared by these regulatory elements.
Key Discoveries: Transcription Factor Motifs and Saft
They discovered three transcription factor motifs—specific DNA sequences that transcription factors bind to in order to regulate gene activity—individually driving silencer activity. One of these motifs, DLM3 (Drosophila Long Motif 3), was computationally discovered a decade ago but had never been characterized before. “Something very interesting that we found is that DLM3 is bound by a previously unknown transcription factor, Saft or Silencer Associated Factor,” explains Hofbauer. “We discovered that Saft helps keep genes turned off in the brain and ovaries, which is crucial for proper development and fertility in flies.”
Silencers appear inaccessible in their native chromatin, where DNA is tightly packed with nucleosomes. However, the researchers show that Saft can locate and bind to its target motifs even in an apparently chromatin state—a unique feature that can explain why it had been so difficult to find silencers before, by approaches that focused on open chromatin.
“The discovery of Saft’s role in gene regulation helps us figure out how silencers work and adds to our understanding of the machinery that controls cell identity and development,” says Hofbauer.
Implications for Broader Research
With a detailed map of silencers now available in Drosophila, researchers can begin to explore whether similar mechanisms operate in other organisms, including humans.
“What we’ve learned here is that to really study these silencers, we need to go in without bias,” says Alexander Stark. “Now that we’ve identified this class of silencers, we can start finding more, dig into how they work, and get closer to fully mapping out the human genome.”
Reference: “A genome-wide screen identifies silencers with distinct chromatin properties and mechanisms of repression” by Lorena Hofbauer, Lisa-Marie Pleyer, Franziska Reiter, Alexander Schleiffer, Anna Vlasova, Leonid Serebreni, Annie Huang and Alexander Stark, 20 November 2024, Molecular Cell.
DOI: 10.1016/j.molcel.2024.10.041
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