#gene repression

Eukaryotes gene regulation

Which enzyme removes acetyl groups during repression?

Histone acetylation neutralizes lysine epsilon-amino positive charge weakening DNA-histone interaction relaxing chromatin structure facilitating transcription factor access. Reversal mediated by histone deacetylases HDACs divided into Rpd3, Hda1, Sir2 families uses zinc or NAD as cofactors restoring positive charge promoting nucleosome compaction and repressive chromatin assembly. During glucose repression Tup1-recruited HDACs Rpd3L, Hda1 deacetylate H3K9, H3K18, H4K16 at GAL promoters preventing SAGA acetyltransferase Gcn5 access. Similarly at Saccharomyces telomeres Sir2 NAD-dependent HDAC deacetylates H4K16 enabling Sir3/Sir4 spreading and telomere position effect. HDAC activity antagonizes HATs establishing dynamic acetylation equilibrium regulating gene expression and heterochromatin maintenance precisely.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 4: Histone Deacetylase HDAC Function in Repression

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Eukaryotes gene regulation

Mig1 represses GAL1 transcription by recruiting which complex?

Mig1 imposes repression by recruiting general co-repressor complex Cyc8-Tup1, yeast equivalent of metazoan TLE/Groucho corepressors. Mig1 C-terminal repression domain interacts with Tup1 WD40 repeat domain assembled with Cyc8 tetratricopeptide motifs stabilizing tetrameric complex. Assembly subsequently attracts histone deacetylases Rpd3L complex containing Hos2, Hda1, inducing deacetylation of H3K9, H3K18, H4K16 at GAL promoters, increasing nucleosome affinity and masking activation domains. Deacetylation reduces accessibility for acetyltransferase Gcn5 within SAGA complex, preventing Gal4 contacting basal machinery despite enhancer occupancy. Tup1-HDAC activity regulates approximately 300 glucose-repressed genes, illustrating leverage of sequence-specific repressor using conserved co-repressor hub mediating metabolic adaptation.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 7: Mig1 Interaction with Tup1-HDAC Complex

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Regulation of chromatin structure

Which histone modification generally leads to gene repression?

Histone lysine methylation status is read by specialized domains that dictate transcriptional outcome depending on residue position and methylation state. Trimethylation of H3 lysine 9 creates binding platform for chromodomain of heterochromatin protein 1, stimulating Suv39h1 recruitment, spreading of repressive domains and interaction with DNA methyltransferases. This enforces chromatin compaction, transcriptional silencing and maintenance of constitutive heterochromatin at pericentromeres and telomeres. In contrast, H3K4me3 and H3K27ac at promoters recruit activating complexes, illustrating residue-specific decoding of methylation into activation versus repression programs.

Ref: NCBI Bookshelf, Biochemistry, Histone Methylation and Heterochromatin, H3K9me3 and HP1 Interaction

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ABCE & ABCDE MODEL +Synapdrgon flowering

RAD represses which gene to control ventral identity?

Pick C: DIV. Thinking through ABCE & ABCDE MODEL +Synapdrgon flowering step by step rules out the lookalikes and leaves DIV. Skip these: A) CYC; B) DICH; D) SEP. For hormones and genes, match the molecule or gene to its real role, not a neighboring pathway.

Ref: Best CSIR NET Plant Physiology books: Master Unit 6 with Taiz & Zeiger and Salisbury & Ross. Crack Part C experimental questions with top textbooks.

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