Bacteria boasts tiny genome, is codependent

Scientists have discovered a bacteria the survives with an incredibly small number of genes:

The tiniest genomes ever found belong to two types of bacteria that live inside insects, researchers have announced.

One of these types of bacteria, Carsonella ruddii, is so small that it could perhaps be considered an organelle within the cells of the bugs. But both microbe species face the threat of extinction because of their small genome size, experts say.

C. ruddii has the fewest genes of any cell known in the world - a mere 182, according to the new results. Humans, by comparison boast approximately 20,000 genes. The entire genome of the microbe contains only 159,622 base pairs (160 kilobases).

"This is on the order of a large virus," says Craig Venter, a gene-entrepreneur and head of the J Craig Venter Institute in Rockville, Maryland, US, who was not involved in this study. He explains that the smallpox virus measures about 200 kilobases in length.

The previous record for the smallest genome belonged to the Mycoplasma genitallium bacterium, which has a total genome size of about 580 kilobases.

From the stories I read covering this, they are advertising this finding as getting closer to the minimum number of genes required for survival. This bacteria would suggest that the generic blank bacterium with the minimum genes for survival is much simpler than previously believed.

But that is a really dumb way to frame the question of what genes do. Evolutionarily what determines whether a gene is necessary is not whether you can knock it out and still get a viable bacteria under some laboratory condition. Rather, genes are necessary only in the context of certain environments. If a bacteria exists in an environment where lactose is its only food, then lactose metabolizing genes are necessary. If there is no lactose, these genes are dispensible.

The researchers of this study mention, but don't really emphasize the fact that this bacterium is present as a parasite in an insect. Parasitism allows it to dispense with certain genes that it would normally need for survival. Now, I am not certain whether this bacteria is an obligate parasite or not -- whether it can survive outside the host -- but where are you going to draw the line? Hell, I can design a bacteria without a cell membrane if I am willing to grow it in a nutritious media where I supply every nutrient it needs. That doesn't mean that the genes for nutrients are dispensible; it just means that I can create a condition where something like life will still exist.

Craig Venter mentioned something similar:

Cell biologists say these findings show how few genes a cell needs to get by: "I think it's important because scientists are trying to understand the minimal parts you need to encode for a cell to function," says Stephen Giovannoni of Oregon State University in Corvallis, Oregon, US.

This information, he adds, will help scientists trying to build artificial cells achieve their goals. Such cells could be used to manufacture products like fuel and antibiotics.

But Venter, who aims to build an artificial bacterium with as few genes as possible, says the use of the new findings is limited because C. ruddii is so dependent on its host. This makes it somewhat irrelevant to his team's efforts to make self-sufficient bacteria. "I don't think it says anything about the genes required for independent cell growth," says Venter.

The one thing that is really interesting about this research is that it supports a possible means of eukaryotic evolution. Eukaryotic cells -- cells in higher, multicellular organisms -- have organelles that looks suspiciously like bacteria -- mitochondria and chloroplasts. It is has been suggested that these were orginally parasitic bacteria that just lost so many genes that they became obligate. This research would certainly support that.

There was good coverage of this story on NPR last night if you want to listen to it.

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Could that discovery suggest a possible path for virus creation ?
I understand viruses are not considered to be an extreme form of "devolved" bacteria.

With regard to chloroplasts and mitochondria, you imply it, but I think it is important to state clearly, that these organelles still have there some of their own DNA. The mitochondrial and chloroplast genes are also more similar to bacterila genes than eukaryotic genes.

I thought that the bacterial origin of mitochondria was kind of settled. Is there still any serious challenge to the theory?

By dileffante (not verified) on 13 Oct 2006 #permalink