Most viruses travel light. They carry only a handful of genes for making new viruses, relying on their hosts' machinery to do the rest. But the newly identified virus, known as the Cafeteria roenbergensis virus, is a pack rat: It lugs around a staggering 730,000 base pairs of DNA, including more than 500 gene-like regions, researchers report  online today in the Proceedings of the National Academy of Sciences. That makes it the largest marine virus known, with more DNA than some bacteria have. The researchers speculate that this virus—which infects Cafeteria roenbergensis, a predatory single-celled organism that eats other viruses and bacteria in the ocean—takes a more active role in making its proteins than do smaller viruses like HIV or herpesviruses. The only known virus that is larger, thought to be a close relative, infects a freshwater amoeba. Scientists typically don't classify viruses as living organisms, but giant viruses like these, with their own protein-making machinery and other functions normally carried out in living cells, blur the lines between what's alive and what isn't.
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