#E2F

Cell Cycle Regulation

The major function of E2F transcription factors is to:

E2F transcription factor family orchestrates G1 to S gene expression program required for DNA synthesis and cell cycle progression. Activator subgroup E2F1, E2F2, E2F3 bound to DP1/2 drives transcription once freed from pocket proteins Rb, p107, p130. Released E2Fs recognize consensus binding site TTTCGCGC in promoters recruiting histone acetyltransferases p300, CBP, PCAF and mediator enabling RNA polymerase II initiation. Transcriptional targets include broad cohort essential for S phase: replication licensing factors ORC1, Cdc6, Cdt1, MCM2 through MCM7 helicase subunits, activation factors Cdc45, Dbf4, Treslin, GINS, DNA polymerases alpha, delta, epsilon, sliding clamp PCNA, flap endonuclease Fen1, nucleotide metabolism dihydrofolate reductase DHFR, thymidylate synthase TYMS, thymidine kinase TK1, checkpoint regulators Chk1, cell cycle regulators Cyclin E, Cyclin A, Skp2, E2F itself and p107 creating feedback. Hence E2F coordinates replication protein supply, origin licensing, nucleotide pools and S phase kinase activators converting G1 cell into replicating entity. Activity limited by Cyclin A-CDK2 phosphorylation causing E2F1 degradation via SCF-Skp2, recruitment of Rb family repressors in late S, and microRNA control to ensure single round synthesis; deregulated activity induces apoptosis via ARF-p53 or oncogenic proliferation.

Ref: Bracken et al., E2F Dependent Transcription and S-Phase Gene Regulation, Trends Biochem Sci 2004; Alberts et al., Chapter 17, E2F Control.

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Cancer

Hyperphosphorylation of Rb leads to

Hypophosphorylated Rb binds and represses E2F transcription factors, preventing expression of genes required for S-phase entry. Mitogenic signaling activates cyclin-D–CDK4/6 and cyclin-E–CDK2 complexes that progressively phosphorylate Rb. Hyperphosphorylated Rb releases E2F, allowing S-phase progression. Loss of Rb function therefore removes a critical G1 checkpoint and contributes to uncontrolled proliferation. Careful

Ref: NCERT Biology Class 11–12 Alberts et al Molecular Biology of the Cell Lodish et al, Molecular Cell Biology Cooper & Hausman, The Cell Abbas et al., Cellular and Molecular Immunology (for immunology sections)

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Cancer

Hypophosphorylated Rb is

Active in blocking cell cycle, is consistent with established principles of cell signaling, receptor pharmacology and cellular regulation. Experimental measurements of binding parameters, genetic loss-of-function studies and pharmacological interventions all converge on the same interpretation. Related options address neighboring concepts but do not satisfy the precise criterion stated in the question.

Ref: NCERT Biology Class 11–12 Alberts et al Molecular Biology of the Cell Lodish et al, Molecular Cell Biology Cooper & Hausman, The Cell Abbas et al., Cellular and Molecular Immunology (for immunology sections)

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Cancer

Rb protein prevents S-phase entry by binding to

Hypophosphorylated Rb binds and represses E2F transcription factors, preventing expression of genes required for S-phase entry. Mitogenic signaling activates cyclin-D–CDK4/6 and cyclin-E–CDK2 complexes that progressively phosphorylate Rb. Hyperphosphorylated Rb releases E2F, allowing S-phase progression. Loss of Rb function therefore removes a critical G1 checkpoint and contributes to uncontrolled proliferation. Careful

Ref: NCERT Biology Class 11–12 Alberts et al Molecular Biology of the Cell Lodish et al, Molecular Cell Biology Cooper & Hausman, The Cell Abbas et al., Cellular and Molecular Immunology (for immunology sections)

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