Soil Microbiology

• Bacteriophage Mu (Mutator Phage): Bacteriophage Mu is a temperate double-stranded DNA bacteriophage that infects E. coli. It is unique because it replicates entirely by replicative transposition.
• Mutator Phenotype: During both lysogenic integration and lytic amplification, Mu inserts its $37\text{ kb}$ genome essentially at random positions throughout the host chromosome via transposase (MuA) and accessory protein (MuB), causing high-frequency insertion mutations ($1-3\%$ of host genes inactivated, hence named 'Mu' for mutator).
• Other Phages:
• Lambda ($\lambda$): Replicates via bidirectional theta replication followed by rolling circle replication, integrating via site-specific recombination at attB.
• T7: Virulent linear dsDNA phage replicating by concatemer formation.
• M13: Filamentous ssDNA phage replicating by rolling circle mechanism without transposition.
• Conclusion: Phage Mu replicates by transposition (Option 3).

Correct option: (3) Mu

• Bacterial Two-Component and Osmoregulatory Systems:
• Statement (B) is correct: The Ntr regulatory system (NtrB sensor kinase / NtrC response regulator, misprinted as 'Mtr' in the exam paper) regulates nitrogen assimilation, activating glutamine synthetase (glnA) under nitrogen limitation.
• Statement (C) is correct: The Nar system (NarX-NarL / NarQ-NarP) detects nitrate and nitrite, activating nitrate reductase (narGHJI) operons while repressing alternative anaerobic pathways (fumarate, DMSO).
• Statement (E) is correct: Osmolarity of the environment controls outer membrane porins through the EnvZ-OmpR two-component system: high osmolarity favors smaller pore OmpC, whereas low osmolarity favors larger pore OmpF.
• Statement (A) is incorrect: Two-component systems are absent or minimal in degenerate bacterial intracellular parasites (e.g., Mycoplasma, Chlamydia), which have drastically reduced genomes.
• Statement (D) is incorrect: Eukaryotic signal transduction relies predominantly on phosphorylation of Serine, Threonine, and Tyrosine residues, whereas bacterial two-component systems use histidine kinases and aspartate receivers.
• Conclusion: Statements (B), (C), and (E) are correct (Option 4).