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Characterization of the Escherichia coli membrane domain responsible for binding oriC DNA
A Chakraborti1, S Gunji, N Shakibai
1Department of Microbiology, University of Connecticut Health Center, Farmington 06030.
Abstract:
It has previously been shown that hemimethylated DNA from the Escherichia coli replication origin (oriC) binds with high specificity to membrane fractions isolated from disrupted cells. In this article, the membrane localization of oriC-binding activity was studied by subjecting crude membrane preparations to successive cycles of sedimentation and flotation gradient analysis. This revealed that approximately two-thirds of the membrane-associated oriC-binding activity of the cell was not associated with the outer membrane fraction as previously suggested but was recovered instead in a unique membrane fraction (OCB1) whose buoyant density and protein profile differed from those of both inner and outer membranes. The specific activity of oriC binding in OCB1 was approximately fivefold higher than the activity of the isolated outer membrane peak. It is likely that membrane fraction OCB1 includes the membrane domain responsible for the binding of hemimethylated oriC to the cell envelope in intact cells.
Insights
Researchers identified a novel membrane fraction (OCB1) in Escherichia coli responsible for binding hemimethylated DNA at the oriC replication origin, challenging previous assumptions about outer membrane involvement.
Area of Science:
- Molecular Biology
- Cellular Biology
- Microbiology
Background:
- Hemimethylated DNA from Escherichia coli replication origin (oriC) exhibits high specificity for membrane fractions.
- Previous studies suggested outer membrane localization for oriC-binding activity.
Purpose of the Study:
- To investigate the precise membrane localization of oriC-binding activity within Escherichia coli.
- To characterize the membrane fraction(s) involved in binding hemimethylated oriC.
Main Methods:
- Crude membrane preparations were subjected to sequential sedimentation and flotation gradient analysis.
- Buoyant density and protein profiles of membrane fractions were analyzed.
Main Results:
- Approximately two-thirds of membrane-associated oriC-binding activity was found in a unique fraction (OCB1), not the outer membrane.
- OCB1 exhibited a distinct buoyant density and protein profile compared to inner and outer membranes.
- OCB1 showed a fivefold higher specific activity for oriC binding than the isolated outer membrane peak.
Conclusions:
- The study identifies a novel membrane fraction (OCB1) as the primary site for hemimethylated oriC binding in E. coli.
- OCB1 likely represents the specific membrane domain responsible for oriC binding to the cell envelope in intact cells.
- Findings challenge previous notions of outer membrane involvement in oriC-DNA interactions.