Gene world's bad guys revamp their image

 作者:倪耨诽     |      日期:2019-03-08 08:17:11
By Philip Cohen A GENETIC vandal, infamous for rampaging through the genome and destroying genes, may after all have played a constructive part in evolution, a new report says. Snippets of DNA called retrotransposons have a bad reputation. They reproduce by making RNA copies of themselves, which then turn back into DNA and slot into new positions in the genome. One particular retrotransposon, called L1, has been busy replicating itself in animal genomes for aeons, so that non-functional copies of L1 now account for about 15 per cent of human DNA. Occasionally, such meandering has caused mutation and disease. But L1 may not be all bad, according to Haig Kazazian and his colleagues at the University of Pennsylvania in Philadelphia. They knew that L1 RNA copies sometimes include neighbouring sequences. If these pieces could be large enough to include an active chunk of a gene called an exon, wandering L1s could copy the exon as well. Such “exon shuffling” is believed to have been a major driving force of evolution because it allows useful fragments of different genes to join up in new ways. To test the idea, Kazazian and his colleagues took an exon containing a gene that confers antibiotic resistance and inserted it into human cells. The gene could become active only if the whole exon had hitchhiked a ride on an adjacent L1 copy, and was then inserted correctly into an active gene. Cells in which this leap actually occurred would be made drug resistant. The team found that about 1 per cent of transfers resulted in drug-resistant colonies. Since genes make up only 15 per cent of human chromosomes, this suggests L1s frequently managed to carry their exon cargo ( Science, vol 283, p 1530). “Now we need to take another look at animal DNA to see if we can find direct evidence of L1’s creative influence,