Complex minimal complexity

According to this site:

In three papers published back-to-back in Science, they provide the first comprehensive picture of a minimal cell, based on an extensive quantitative study of the biology of the bacterium that causes atypical pneumonia, Mycoplasma pneumoniae. The study uncovers fascinating novelties relevant to bacterial biology and shows that even the simplest of cells is more complex than expected.


A network of research groups at EMBL’s Structural and Computational Biology Unit and CRG’s EMBL-CRG Systems Biology Partnership Unit approached the bacterium at three different levels. One team of scientists described M. pneumoniae‘s transcriptome, identifying all the RNA molecules, or transcripts, produced from its DNA, under various environmental conditions. Another defined all the metabolic reactions that occurred in it, collectively known as its metabolome, under the same conditions. A third team identified every multi-protein complex the bacterium produced, thus characterising its proteome organisation.

“At all three levels, we found M. pneumoniae was more complex than we expected,” says Luis Serrano, co-initiator of the project at EMBL and now head of the Systems Biology Department at CRG.

So in what ways is this minimal cell more complex than expected?

One way is this:

When studying both its proteome and its metabolome, the scientists found many molecules were multifunctional, with metabolic enzymes catalyzing multiple reactions, and other proteins each taking part in more than one protein complex. They also found that M. pneumoniae couples biological processes in space and time, with the pieces of cellular machinery involved in two consecutive steps in a biological process often being assembled together.

Multifunctional proteins?  Recall that in The Design Matrix, I wrote:

The list of moonlighting proteins is quite impressive and getting larger every day. Ramasarma provides a list of fifty-six examples.36 What is remarkable is that there is no solid method or test for identifying moonlighting proteins, as scientists have largely discovered them through serendipity. The hypothesis of front-loading would predict these fifty-six examples are just the tip of the iceberg. In fact, because of their telic utility, a front-loading perspective predicts that multi-functional proteins will turn out to be commonplace.

Well, it looks like a systems biology approach may have uncovered more of this iceberg.  And recall also that I used the logic of front-loading to predict that most ribosomal proteins would be multifunctional.   Now, while I have not read the Science papers yet, check out the figure at the ScienceDaily site.  It shows a ribosome with streams of yellow lines connected to it.  The legend of this figure explains them as follows:

Apart from these expected interactions, the scientists found that, surprisingly, many proteins are multifunctional. For instance, there were various unexpected physical interactions (yellow lines) between proteins and the subunits that form the ribosome, which is depicted as an Electron microscopy image (yellow). (emphasis added)


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