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#Poly(A) tail

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Transcription in Eukaryotes

Poly(A) tail is added by

After cleavage at polyadenylation site, polyadenine tail is synthesized not by RNA polymerase II but by specialized nuclear polyadenine polymerase PAPOLA and PAPOLG, recruited through interaction with CPSF via FIP1 subunit and stimulated by nuclear polyadenine binding protein PABPN1. Enzyme is template independent, uses ATP to add adenylates processively: initial distributive addition of ten to twelve adenines, then PABPN1-dependent switch to processive synthesis of two hundred to two hundred fifty residues in mammals. Polyadenine tail enhances stability, export, and translation via PABPC and eIF4G circularization bridge.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 6: Poly(A) polymerase adds tail; Lodish 9th ed., Mechanism of polyadenylation

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Acid, Base, pH

Poly(A) tail assists translation by binding

PABP accurately identifies the binding site, binding partner, or molecular interaction described in this question. In Acid, Base, pH, molecular recognition and binding specificity are governed by complementary shape, charge, and hydrophobic interactions between molecules. PABP binds at the specified location due to its structural complementarity and specific non-covalent or covalent interactions. The other options (RNA pol II, ribosome directly, and tRNA) describe binding to different sites, involve different types of molecular interactions, or represent incorrect binding partners.

Ref: Lehninger Principles of Biochemistry, Nelson & Cox, 8th Ed., Ch. 2

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