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Why gene hunters need hot springs

作者:寇吏零    发布时间:2019-02-28 08:18:01    

By Andy Coghlan AN ARTIFICIAL enzyme derived from bacteria in hot springs will enable researchers to sequence DNA much faster than they can now. The enzyme should reduce the time it takes to identify genes responsible for causing disease and to decipher the human genome. DNA is built from a four-letter alphabet of bases – adenine, guanine, cytosine and thymine. The new enzyme is designed to improve a technique called Sanger cycle sequencing, which is already used to “read” along the DNA to discover the order of these bases. The technique relies on the fact that double strands of DNA are linked by specific pairings of bases: thymine only binds to adenine and guanine only pairs with cytosine. So if the sequence of one strand is known, the sequence of the other can be inferred. The researchers use their unknown strand – which might be 1000 bases long – as a template, and build up complementary DNA fragments of every length between 1 and 1000 bases. By analysing these fragments, they can then identify the order of the bases. Today, this procedure has to be repeated up to 10 times before researchers can be sure that they have the correct sequence of bases because existing enzymes are unreliable. Some react at different rates with different bases, yielding ambiguous results, while others are destroyed by the high temperatures needed for cycle sequencing. Stanley Tabor and Charles Richardson at Harvard Medical School in Boston say that their artificial enzyme will overcome both problems. Because their enzyme was engineered from a natural protein produced by bacteria that live in hot springs, it could already survive high temperatures. The researchers then altered it to react at the same rate with all of the bases. In cycle sequencing, researchers start by chopping a DNA sample into fragments of about 1000 bases long and separating the two strands. They then add a mixture of the four bases in solution. Tabor and Richardson’s enzyme encourages these free bases to attach one by one to the single strand of DNA – reinventing the complementary strand. To ensure that the second strand always begins at the same point on the DNA, researchers first attach a short stretch of DNA, known as a primer, to the original strand. They also chemically modify some of the free bases to make “chain terminators”. Once these attach to the first strand they halt construction of the second. The enzyme attaches the chain terminators at random positions along the original strand, creating every possible complementary fragment between 1 and 1000 bases long in equal quantities. The researchers then unzip the complementary fragments from the original strands by heating them up. They separate all the fragments by weight, and identify the terminator base on each one. The most common way to do this is to attach a different dye to each of the four types of terminator. Every fragment of a certain length is then “colour-coded” depending on whether its terminal base is guanine, cytosine, adenine or thymine. Tabor and Richardson’s findings are published this month in the Proceedings of the National Academy of Sciences (Vol 92, 3 July). The British company Amersham Life Science will market the enzyme as Thermo Sequenase. The enzyme should be of real value in large-scale tasks such as sequencing the human genome, says André Rosenthal of the Institute of Molecular Biology at Jena in Germany, who has tested the compound. “The enzyme provides more data per experiment,

 

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