Visualization of prokaryotic DNA in a regularly condensed chromatin-like fiber

Area of Science:

• Microbiology
• Molecular Biology
• Electron Microscopy

Background:

• Escherichia coli (E. coli) is a key model organism in molecular biology.
• Bacteriophages, like lambda phage, are viruses that infect bacteria and have complex DNA structures.
• Understanding bacterial cell envelope and DNA organization is crucial for various biological processes.

Purpose of the Study:

• To investigate the structure and composition of fibers observed in disrupted Escherichia coli cells.
• To determine the relationship between these fibers and the DNA of superinfecting lambda phage.

Main Methods:

• Electron microscopy was used to examine disrupted Escherichia coli cells.
• Specific lysis and dehydration protocols were employed to preserve cellular structures.
• Lysogenic E. coli cells were superinfected with lambda phage (wild type and mutants).

Main Results:

• Fibers approximately 120 A in diameter with a 130 A repeating beaded substructure were observed attached to the cell envelope.
• These fibers were only detected under specific lysis and rapid dehydration conditions.
• Short loops of these fibers, free from the cell envelope, were observed in superinfected cells.
• The length of these fiber loops correlated directly with the DNA content of the superinfecting lambda phage.

Conclusions:

• The observed fibers are composed of DNA condensed approximately 6.5-fold.
• The DNA is organized in blocks of about 250 base pairs within these fibers.
• This study provides insights into DNA packaging and organization within bacterial cells and phage systems.