Q.4 The role of enzyme E synthesized by phage X174 during host infection is to
(A) block peptidoglycan synthesis
(B) enhance synthesis of viral +RNA
(C) inhibit lipid metabolism
(D) stimulate dsDNA replication
The correct answer is (A) block peptidoglycan synthesis.
Phage φX174, a single-stranded DNA bacteriophage, infects E. coli and relies on enzyme E (also called protein E), a 91-residue membrane protein, to trigger host cell lysis late in its replication cycle. This protein specifically targets MraY, an essential enzyme in peptidoglycan biosynthesis, halting cell wall precursor (lipid I) formation and causing growth-dependent lysis.
Option Analysis
(A) Block peptidoglycan synthesis
This is correct. Enzyme E inhibits MraY by inserting its transmembrane helix into the enzyme’s lipid substrate-binding groove, forming the YES complex (with MraY and SlyD chaperone) that blocks peptidoglycan precursor synthesis, leading to cell wall defects and lysis.
(B) Enhance synthesis of viral +RNA
Incorrect. φX174 has a DNA genome, not RNA, so no +RNA synthesis occurs; gene A protein and host enzymes handle DNA replication stages instead.
(C) Inhibit lipid metabolism
Incorrect. While E interacts with membrane lipids via MraY, its primary role targets peptidoglycan synthesis specifically, not general lipid metabolism.
(D) Stimulate dsDNA replication
Incorrect. dsDNA replicative form (RF) synthesis uses phage gene A protein, host Rep helicase, and DNA polymerase III; enzyme E acts only for lysis, not replication.
Phage φX174, a model ssDNA bacteriophage infecting E. coli, synthesizes enzyme E during late-stage host infection to ensure progeny release by blocking peptidoglycan synthesis. This 91-amino-acid lysis protein exemplifies single-gene lysis strategies in microviruses, crucial for CSIR NET Life Sciences preparation.
φX174 Life Cycle Overview
φX174 enters via LPS receptors, converts ssDNA to dsRF using host enzymes, replicates via rolling-circle mechanism, and assembles ~100 progeny phages. Late expression of gene E halts MraY activity in peptidoglycan synthesis, causing septal lysis without holins or endolysins.
Mechanism of Enzyme E Action
Enzyme E’s N-terminal transmembrane helix binds MraY’s active site, kinking via conserved proline to obstruct undecaprenol phosphate access, preventing lipid I formation. SlyD stabilizes the YES complex, enhancing inhibition; mutants confirm specificity. Cryo-EM structures validate this noncompetitive block.
Relevance to Competitive Exams
In CSIR NET, such questions test phage biology integration with bacterial cell wall synthesis. Option (A) aligns with evidence from genetic screens and structures; others confuse with RNA phages or replication proteins.
