Of all the life forms inhabiting planet Earth - whether animal, vegetable, mineral, mammal, bird, insect, virus, bacterium, whatever - the living being with the smallest genome ever found is a microbe that made its online bow Oct. 13, 2003, in the Proceedings of the National Academy of Sciences (PNAS).

The report is titled: "The genome of Nanoarchaeum equitans: Insights into early archael evolution and derived parasitism."

"In the paper we are three corresponding contributors," explained its lead author, molecular biophysicist and biochemist Dieter Soll at Yale University in New Haven, Conn. "The senior author is Michiel Noordewier, at Diversa Corp., of San Diego. The second one is microbiologist Karl Stetter at the University of Regensburg in Germany, and the third one is me, Dieter Soll at Yale. The significance of this topical paper," Soll told BioWorld Today, "is now very clear: It's the neat organism that represents a new living kingdom."

"The analysis of this genome sequence," Diversa's Noordewier observed, "provides insight into the simplest known metabolism required by an organism for survival. It supports theories that life originated in a hot and fiery primeval earth. The N. equitans genome," he added, "comprises 490,885 base pairs, the smallest microbial genome sequenced to date. Our co-author, Karl Stetter, reported its discovery last year [in Nature, dated May 2, 2002]. To investigate hot submarine vent microbial communities,' he wrote, we carried out experiments to cultivate hyperthermophiles from samples of originally hot rocks and gravel taken at the Kolbeinsey ridge, north of Iceland.'"

"Similar thermal pools," Soll noted, "have been found in Kamchatka and also in Yellowstone National Park. The different colors in Yellowstone pools are all caused by varicolored bacteria, depending on their different temperatures. These range above 90 degrees Celsius, or about 200 degrees Fahrenheit."

Never Will N. equitans Part From Ignicoccus

Despite its compact, close-to-the-chest genomic contour, N. equitans is far from a lone player. Rather, it's a shameless parasite, as Soll explained, calling it "an obligate symbiont growing in co-culture with the crenarchaeon Ignicoccus. How it actually lives with its host organism is not understood currently, but clearly it shows that N. equitans' symbiotic relationship to Ignicoccus is parasitic, making it the only known archael parasite. The organism must receive a vast amount of cellular metabolized material from its host. Otherwise it wouldn't be able to form lipids or nucleic acids, and so on.

"So the reason we in my lab were interested in the Nanoarchaeum," Soll went on, "was the fact that there were a reasonably high number of so-called split genes. As Figure 2 of the PNAS paper shows on page 4, the very large RNA and DNA polymerases are not encoded as a single piece, but as two fragments. We did an experiment to see whether the two parts are independently active or not active when they are expressed by themselves. But the two proteins that we mixed became very active.

"We are really excited" Soll said, "by the occurrence of these split genes. So now in our ongoing research, we are looking at the chemical and enzymatic properties of a number of these split genes to see whether these nonarchael organisms could achieve something that other organisms obtain only with lots of protein or small DNA or something like it. So our hope is that we achieve some minimal-type enzyme or activity that has to be on this organism because there's so little DNA in it, while in other organisms there is more. The genes could be larger, and they would have more room to play.

"Can we find in these or other organisms the metabolic pathways compared to the ones that we know from yeast or from human cells?" Soll asked rhetorically. "These microorganisms live on sulfur or hydrogen or ammonia or chemicals we cannot live on or use. These keep enzymatic reactions that we no longer have in mammalian cells and as such might be of interest for biotechnological application, because certainly when we deal with those other compounds, say from mammalian cells, we wouldn't know how to transform it into something else."

If It Were Only A Virus, Which It's Not

"From my perspective," Diversa's Noordewier said, "that microbe has none of the metabolic complements. On the other hand, it does have a complete informational process system. So it can replicate its DNA, translate and transcribe. I think that has implications for the lifestyle of the organism that are quite intriguing.

"The features we see, such as lack of operons, the large number of split genes, the lack of pseudogenes, imply that its archeolineage is very ancient in its origins. This organism cannot possibly live on its own without Ignicoccus. What distinguishes it from a virus? No virus yet discovered has ribosomal machinery for translating and repair of proteins. On the other hand it contains nothing you would expect from a free-living organism, even from such as mycoplasma or other known parasites.

"There is no direct immediate medical benefit," he pointed out, "but there are scientific advantages. For one thing, N. equitans is a hyperthermophile. We should be able to study this relatively simple molecule and learn how certain functions support its lifestyle, particularly its survival under these extreme temperatures. Finally, considering that it's a minimal genome, the smallest yet found and described, the scientific results that we expect [could help us] learn about what it might take to make another minimal organism in order to get to the same results. I think," he concluded, "that would have the most relation to the question, what are the medical or industrial uses of this organism, to give us insights into what it takes to minimalize a genome.

"So, this is an organism either on its way into the cell or on its way out. What we're seeing is an organism that is becoming more and more inextricably bound to this organism, or on its way to becoming more free living. It sometimes straddles a lifestyle niche that we have not yet seen before. If it were a virus we would have a working set of hypotheses as to how it works. But it's not," he concluded.