Processive action of terminase during sequential packaging of bacteriophage lambda chromosomes

Area of Science:

• Molecular Biology
• Virology
• Genetics

Background:

• Bacteriophage lambda chromosomes are packaged sequentially from multimeric DNA.
• Terminase enzyme nicks cohesive end sites (cos) to generate mature chromosomes.
• Previous models suggested terminase scans for terminal cosN to terminate packaging.

Purpose of the Study:

• To investigate the processive action of terminase during sequential packaging of bacteriophage lambda chromosomes.
• To determine if terminase remains bound to DNA after initial packaging to facilitate subsequent packaging events.

Main Methods:

• Helper packaging experiments using trilysogens (strains with three tandem prophages).
• Construction of multilysogens with modified cos sites (cos+ and cosB-) to test terminase binding and scanning.
• Analysis of packaging efficiency with different cos site configurations.

Main Results:

• Two chromosomes can be packaged from trilysogens even when the central cos site lacks cosB.
• Terminase remains bound to the distal DNA after cleaving the central cos, initiating downstream packaging.
• Terminase can initiate and terminate packaging of downstream chromosomes, with a 0.3-0.5 frequency of reading across cos sites.

Conclusions:

• Terminase exhibits processive action, remaining bound to DNA to facilitate sequential packaging of bacteriophage lambda chromosomes.
• The binding of terminase to cosB is primarily required for initiating the first chromosome's packaging.
• A model of processive terminase action explains the sequential packaging mechanism of lambda chromosomes.