Japanese Researchers Enhance DNA Assembly Efficiency Using Silver Nanoparticles

Researchers from Nagoya University and Gifu University in Japan have developed a novel method that utilizes silver nanoparticles for cutting and rejoining DNA at specific sites, achieving DNA assembly efficiencies two to five times greater than traditional restriction enzyme methods. Their findings were recently published in the journal Nucleic Acids Research.

Challenges with Conventional DNA Assembly

Standard DNA assembly processes typically rely on restriction enzymes to make precise cuts in DNA sequences and T4 DNA ligase to connect the resulting fragments. However, these enzymes have notable limitations; they can only recognize specific DNA sequences and often produce short sticky ends, which hinder the efficiency of fragment joining.

A team led by Professor Hiroshi Abe and Assistant Professor Masahito Inagaki aimed to address these challenges by exploring the potential of chemical reactions to cut DNA instead of relying solely on restriction enzymes. Their investigation revisited a previously reported method from the early 1990s, where silver ions were shown to cut 3′-thiol-modified DNA. Although effective in making cuts, silver ions caused non-specific attachment and precipitation, resulting in a DNA recovery rate of only 14%. This was deemed insufficient for practical applications.

Improved Efficiency with Silver Nanoparticles

The researchers shifted from using silver ions to silver nanoparticles, which they believed could be easily separated from the reaction mixture via centrifugation, thereby improving DNA recovery. Initial experiments demonstrated nearly 100% cleavage efficiency at high temperatures, but such conditions could damage longer DNA strands.

To tackle this issue, the nanoparticles were coated with polyethylene glycol (PEG), enhancing their stability and dispersion. This modification resulted in a significant increase in DNA cleavage efficiency, from 36% to 92%, under more moderate conditions—37°C over 31 hours. Inagaki, the study’s first author, noted that this optimization finally achieved a PEG-modified cleaving efficiency exceeding 91% at 50°C in just one to two hours.

A further advantage of this nanoparticle approach was its ability to separate unwanted DNA fragments, enhancing the recovery rate from 14% to 98% after purification.

Enhancements in Sticky Ends and Joining Efficiency

The use of silver nanoparticles also enabled the production of DNA fragments with longer sticky ends—up to 8 bases—providing a significant advantage over conventional methods, which struggle to produce such extensions. With T4 DNA ligase, the connecting efficiency for these longer fragments was nearly doubled compared to traditional techniques.

Using an 18-base sticky end, researchers achieved a joining efficiency of 44%, a stark improvement over the 8% efficiency associated with conventional 4-base ends.

Practical Applications and Future Directions

To assess the practical applicability of their method, the team successfully introduced a DNA fragment encoding green fluorescent protein (GFP) into human HeLa cells, confirming accurate assembly through GFP expression.

Inagaki remarked on the broader implications of this technology, highlighting its potential utility in synthesizing genomic DNA for various applications, including mRNA libraries for cancer vaccines, gene therapy, and the development of artificial protein drugs and genome-edited crops. The research team now aims to determine if their technique can be expanded to join multiple DNA fragments simultaneously, which is crucial for constructing genome-scale DNA.

This research received support from the Japan Science and Technology Agency (JST), the Japan Agency for Medical Research and Development (AMED), and the Tanaka Kikinzoku Memorial Foundation.