Human bromodomain and extra-terminal domain (BET) family members are promising targets for therapy of cancer and immunoinflammatory diseases, but their mechanisms of action and functional redundancies are poorly understood. Bdf1/2, yeast homologues of the human BET factors, were previously proposed to target transcription factor TFIID to acetylated histone H4, analogous to bromodomains that are present within the largest subunit of metazoan TFIID. We investigated the genome-wide roles of Bdf1/2 and found that their important contributions to transcription extend beyond TFIID function as transcription of many genes is more sensitive to Bdf1/2 than to TFIID depletion. Bdf1/2 co-occupy the majority of yeast promoters and affect preinitiation complex formation through recruitment of TFIID, Mediator, and basal transcription factors to chromatin. Surprisingly, we discovered that hypersensitivity of genes to Bdf1/2 depletion results from combined defects in transcription initiation and early elongation, a striking functional similarity to human BET proteins, most notably Brd4. Our results establish Bdf1/2 as critical for yeast transcription and provide important mechanistic insights into the function of BET proteins in all eukaryotes.
Keywords: BET proteins; S. cerevisiae; TFIID; bromodomain; chromosomes; gene expression; genetics; genomics; mediator; transcription elongation; transcription initiation.
When a healthy cell creates new proteins, it activates a standard two-step biological manufacturing process. Firstly, DNA is transcribed from a specific gene to generate a strand of messenger RNA, or mRNA. Next, this mRNA molecule is translated to create the final protein product. This process of converting DNA into mRNA is supported by a series of helper proteins, including proteins from the bromodomain and extra-terminal domain (BET) family. Cancer cells can become ‘addicted’ to the process of converting DNA into RNA, leading to the overproduction of mRNA molecules, uncontrolled cell growth and tumor formation. Knocking out BET helper proteins could potentially bring cancer cells under control by halting transcription and preventing tumor growth. However, the precise ways in which BET helper proteins regulate transcription are currently poorly understood, and therefore developing rational ways to target them is a challenge. Building on their previous work, Donczew and Hahn have investigated how two BET helper proteins, Bdf1 and Bdf2, help to regulate transcription in budding yeast. Using a range of genomic techniques, Donczew and Hahn found that Bdf1 and Bdf2 had important roles for initiating transcription and elongating mRNA molecules. Both BET proteins were also involved in recruiting other protein factors to help with the transcription process, including TFIID and Mediator. Based on these findings, it is likely that cooperation between BET proteins, TFIID and Mediator represents a common pathway through which gene expression is regulated across all eukaryotic organisms. Both Bdf1 and Bdf2 were also found to provide the same functions in yeast as similar BET proteins in humans. Using this robust yeast model system to perform further detailed studies of BET factors could therefore provide highly relevant information to expand our understanding of human biology and disease. Ultimately, this research provides important insights into how two members of the BET family of helper proteins contribute to the control of transcription in yeast. This information could be used to guide the design of new drugs for cancer therapy that target not only BET proteins themselves but also other proteins they recruit, including TFIID and Mediator. Such targeted drugs would be expected to be more harmful for cancer cells than for healthy cells, which could reduce unwanted side effects.
© 2021, Donczew and Hahn.