Minimal Cell More Complex Than Expected

Craig Venter’s lab has been working on an interesting project in theoretical biology: what is the minimum set of genes needed for life?  They have taken one of the simplest organisms, Mycoplasma genitalium, and knocked out genes to see which ones are essential and which are nonessential for viability.  (This is part of the “top down” approach to understanding the origin of life; the “bottom up” approach, by contrast, tries to build life from scratch).  Their latest results, published in PNAS,¹ showed a larger number of essential genes – 347 – than their earlier prediction in 1999.  That’s 79% of the organism’s inventory. 

This is a significantly greater number of essential genes than the 265-350 predicted in our previous study of M. genitalium, or in the gene knockoutdisruption study that identified 279 essential genes in Bacillus subtilis, which is a more conventional bacterium from the same Firmicutes taxon as M. genitalium.  Similarly, our finding of 387 essential protein-coding genes greatly exceeds theoretical projections of how many genes comprise a minimal genome such as Mushegian and Koonin’s 256 genes shared by both H. influenzae and M. genitalium, and the 206-gene core of a minimal bacterial gene set proposed by Gil et al.  One of the surprises about the essential gene set is its inclusion of 110 hypothetical proteins and proteins of unknown function.  Some of these genes likely encode enzymes with activities reported in M. genitalium, such as transaldolase, but for which no gene has yet been annotated.   (Emphasis added in all quotes.)

Since this organism, an obligate human parasite, is apparently stripped down to bare essentials, “it is likely that all its 482 protein-coding genes are in some way necessary for effective growth,” they said.  The team hopes this information will lead to building synthetic free-living cells.

¹Glass, Venter et al., “Essential genes of a minimal bacterium,” Proceedings of the National Academy of Sciences USA, Published online before print January 3, 2006, 10.1073/pnas.0510013103.

Wow – that’s a lot of genes, for a lot of proteins and cellular functions.  The authors’ charts of gene networks look like complex flowcharts designed by an engineer.  Since getting one protein by chance is so improbable it would never happen in countless universes (see online book), expecting to get 387 is overkill on evolutionary theories, kind of like H-bombing a city a googolplex number of times.  After awhile, it becomes ridiculous to expect to find any survivors.
    What’s also interesting are the 100 or so nonessential genes.  How could these evolve?  If they do not have survival value, why would nature select them?  Darwinism’s criterion of survival value appears to be too reductionist to explain many aspects of life that, while not essential, add some seasoning to life.