The team reconstructed the genomes of three distinct lineages of ARMAN and found them to be tiny a mere 1 million base pairs, in contrast to hundreds of billions in humans. In the smallest of the three, the average gene length is only 774 base pairs, in contrast to the average gene length in humans of 10,000 to 15,000 base pairs. Base pairs, the smallest chemical units of the gene, are nucleic acids that come in four forms. The base pairs are chained together to make DNA, and a gene is a sequence of base-pairs coding for a unique protein.
The genomes are so small that the researchers initially suspected that the ARMAN microbes are parasites upon other microbes, since parasites can afford to lose genes that their host already has.
But of the 70 individual specimens so far imaged in 3-D, 90 percent seem to be free-living. The other 10 percent are impaled on the mysterious needle-like spines of Thermoplasmatales, the other Archaea living alongside ARMAN in the mine. The researchers suspect that the penetrating spines may mean that the microbes live off other microbes at least part of the time, unlike symbiotic organisms or parasites, which must always associate with other organisms to live.
"ARMAN are among the smallest microbes we know of that, if not free-living, are at least not permanently obliged to be a parasite or symbiont," Comolli said.
The cells are about as large as the largest viruses, which can replicate only in living organisms and are not considered to be "living."
"The genome is very compact," Baker added." A microbial genome 10 percent larger has the same number of genes as ARMAN."
The organism has a much higher percentage 45 percent of unknown genes than any other organism sequenced, he said.
|Contact: Robert Sanders|
University of California - Berkeley