New technique could help scientists creat custom genes in twenty-four hours

For the lazy:

Researchers have been able to make DNA since the 1970s. The traditional approach takes DNA nucleotides—the chemical letters A, G, C, and T—and adds them, one by one, to a growing chain called an oligonucleotide, or oligo. But the process, which uses a series of toxic organic reagents, is typically slow and error-prone, limiting oligos to about 200 letters—a tiny fraction of the thousands of letters that make up most genes.

Our cells make DNA differently. A variety of enzymes called polymerases read a single strand of DNA and then synthesize a complementary strand that binds to it. That has prompted dreams of re-engineering polymerases to write new DNA.

Over the decades, most researchers have settled on one particular polymerase, called terminal deoxynucleotidyl transferase (TdT), because, unlike other polymerases, it can attach new nucleotides to an oligo strand without following a DNA template strand. Natural TdT does this to write millions of new variations of genes for antibodies, which the immune system can then select from to target invaders. But the natural enzyme adds new DNA letters randomly, rather than controlling the precise sequence of letters as researchers want to do.

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Eventually, the pair settled on a novel approach. They start with four separate pools for the four separate bases, each one with copies of TdT tethered to either A, G, C, or T. To grow their oligo, they add a base from one of the pools. After TdT adds the base to the end of the oligo, it remains tethered, blocking any additional copies of the enzyme from reacting with the oligo and extending it further. The now oligos are then fished out, and the tethers are snipped off. The free TdT is washed away, and the oligo is ready for the next base to be added.

Ultimately, the approach should be cheap, Keasling says, because TdT is easy to manufacture in bacteria and yeast. It’s also fast. Most new nucleotides attach to the growing oligo in 10 to 20 seconds, Palluk, Arlow, Keasling, and their colleagues report today in Nature Biotechnology. For now, the tether snipping step still takes a minute. So synthesizing a whole gene will still likely take the better part of a day.