77. A polypeptide antibiotic isolated from the genus Streptomyces which intercalates into DNA, preventing the progression of RNA polymerase(s) and initiation of DNA replication both prokaryotes and eukaryotes is

(1) Rifampicin         (2) α-Aminitin

(3) Actinomycin D   (4) Cyclohexamide

Introduction

Actinomycin D is a polypeptide antibiotic isolated from the genus Streptomyces that exhibits potent anticancer and antibacterial properties. Its primary mode of action involves intercalation into DNA, which disrupts essential cellular processes such as transcription and DNA replication in both prokaryotic and eukaryotic organisms. This unique mechanism makes it a valuable tool in molecular biology and chemotherapy.

Mechanism of Action

• DNA Intercalation:
  Actinomycin D binds tightly to DNA by inserting its planar phenoxazone ring between adjacent base pairs, especially at GpC sequences. This intercalation distorts the DNA helix, causing unwinding and bending.

• Inhibition of RNA Polymerase:
  By binding DNA, actinomycin D physically blocks the movement of RNA polymerase along the DNA template, preventing transcription elongation. This inhibition halts mRNA synthesis, effectively shutting down protein production.

• Inhibition of DNA Replication:
  The intercalation also impedes the progression of DNA polymerases, thereby inhibiting DNA replication initiation and elongation in both prokaryotes and eukaryotes.

Why Actinomycin D Fits the Description

• It is a polypeptide antibiotic derived from Streptomyces.

• It intercalates into DNA, unlike rifampicin which targets bacterial RNA polymerase without intercalation.

• It blocks both transcription and DNA replication by physically obstructing polymerase progression.

• It acts on both prokaryotic and eukaryotic cells, consistent with its use as an anticancer drug.

Comparison with Other Options

Final Answer

(3) Actinomycin D

Keywords

actinomycin D, DNA intercalation, RNA polymerase inhibition, DNA replication inhibition, polypeptide antibiotic, Streptomyces antibiotic, transcription blocker, anticancer drug, prokaryotic and eukaryotic transcription, DNA polymerase inhibition

Conclusion

Actinomycin D is the polypeptide antibiotic from Streptomyces that intercalates into DNA and prevents the progression of RNA polymerase as well as initiation of DNA replication in both prokaryotic and eukaryotic cells. Its unique mechanism distinguishes it from other antibiotics like rifampicin and α-amanitin, making it the correct choice for the described function.

Correct option:
(3) Actinomycin D