Covalently closed minicircular DNA associated with Acetabularia chloroplasts

Area of Science:

• Molecular Biology
• Plant Cell Biology
• Genetics

Background:

• Chloroplasts contain their own DNA, which can exist in various forms.
• Previous studies identified large circular DNA molecules in chloroplasts of other species.
• The specific structure and origin of Acetabularia cliftonii chloroplast DNA were not fully characterized.

Purpose of the Study:

• To characterize the DNA molecules present in Acetabularia cliftonii chloroplasts.
• To investigate the presence and nature of circular DNA within these chloroplasts.
• To determine the origin of identified DNA molecules.

Main Methods:

• Ethidium bromide-CsCl density gradient centrifugation of chloroplast DNA.
• Measurement of DNA molecule sizes in closed and open conformations.
• Analysis to rule out nuclear or bacterial origins.

Main Results:

• A lower band in the gradient was enriched for DNA (buoyant density 1.712 g/cm3) containing small covalently closed circular molecules (minicircles).
• Minicircle sizes were measured at approximately 4.15 µm (closed) and 4.35 µm (open).
• These minicircles were not of nuclear or bacterial origin, suggesting independent existence within the chloroplast, though mitochondrial origin was not entirely excluded. Large circular molecules (40-45 µm) typical of other chloroplasts were absent. Linear molecules up to 200 µm were observed.

Conclusions:

• Acetabularia cliftonii chloroplasts contain unique small circular DNA minicircles.
• The main chloroplast genome likely consists of very large circular molecules susceptible to shearing.
• Further research is needed to confirm the precise origin and function of these minicircles.