In bacteria, negatively supercoiled chromosomal DNA is organized by nucleoid-associated proteins. Which set best represents such organizing proteins?
Correct Answer: Both HU and H-NS
Introduction / Context:Bacterial chromosomes are compacted without canonical eukaryotic histones. Instead, nucleoid-associated proteins (NAPs) shape and organize negatively supercoiled DNA to enable replication, transcription, and segregation in a crowded cytoplasm.
Given Data / Assumptions:
- We are considering bacterial chromosomal DNA (prokaryotic nucleoid).
- DNA is negatively supercoiled in vivo.
- Proteins that bend, bridge, or wrap DNA contribute to nucleoid architecture.
Concept / Approach:Among the best-studied NAPs are HU (a small, abundant DNA-bending protein) and H-NS (a global gene-silencing/bridging protein that prefers AT-rich DNA). Together, they compact DNA, stabilize supercoils, and help regulate transcriptional programs in response to environmental cues.
Step-by-Step Solution:
Identify which proteins are bona fide bacterial NAPs: HU and H-NS qualify.Exclude typical eukaryotic histones (e.g., histone H1) that form nucleosomes in eukaryotes, not bacteria.Exclude heat-shock chaperones (e.g., HSP-family), which assist protein folding rather than directly organizing DNA.Therefore, the set that best represents organizing proteins is HU and H-NS together.Verification / Alternative check:Genetic deletions and biochemical reconstitution show HU induces flexible bends, while H-NS oligomerizes to bridge duplexes; both strongly influence nucleoid structure and gene expression.
Why Other Options Are Wrong:
- HU only: incomplete; other major NAPs like H-NS also organize DNA.
- H-NS only: incomplete for the same reason.
- Histone H1: eukaryotic linker histone; not a bacterial NAP.
- HSP-1: chaperone, not a DNA-organizing NAP.
Common Pitfalls:Equating bacterial DNA proteins with eukaryotic histones, or assuming any DNA-binding protein is a structural organizer.
Final Answer:Both HU and H-NS