New Life Form Engineered: Answers Questions about LUCA (Last Universal Common Ancestor)

Mixed membranes

In technical terms: the lipids in the membrane of bacteria are made up of straight-chain fatty acids that are ester-linked to a backbone of glycerol-3-phosphate. But the lipids in the archaeal membrane have a backbone of glycerol-1-phosphate, to which isoprenoids are linked by ether bonds.

The idea behind the lipid divide is that a common ancestor of both bacteria and archaea had a cell membrane in which both types of lipid were mixed. ‘This mixed membrane would be less stable than a homogenous membrane of just one type of phospholipid, so eventually a split occurred, resulting in the two domains of Bacteria and Archaea’, says Driessen.

All this would have happened over 3.5 billion years ago, so we can’t expect any hard evidence to be around. Driessen and his co-workers therefore decided to ‘reverse-engineer’ a micro-organism with a mixed membrane. ‘This has been tried before, but these experiments resulted in bacteria with only very small amounts (< 1%) of archaeal lipids.’

Breakthroughs

However, in the new study, this increased to 30 percent. Two key breakthroughs made this possible: ‘In previous research we discovered an enzyme which is crucial to the production of archaeal membrane lipids. This takes three steps, and before that only two of the enzymes involved were known.’ The other breakthrough came from scientists at Wageningen University, explains Driessen. ‘They managed to increase the production of isoprenoids in the bacterium E. coli.’ Isoprenoids are a ubiquitous class of organic compounds which includes many natural flavours, colours or fragrances.

 

Both discoveries were transferred to a normal E. coli bacterium, which was a major feat of genetic engineering. ‘And we didn’t know if the end result would be a viable cell’, says Driessen. But in the end, it worked out well. With some fine tuning, the scientists created a cell in which all phosphatidylglycerol, the lipids which form the basic bilayer of the bacterial membrane, were replaced by their archaeal equivalent (archaetidylglycerol). This accounts for 30 percent of the lipids in the membrane. ‘This bacterium grew at normal speed and was stable’, says Driessen. ‘So this result does not support the hypothesis that a mixed membrane is inherently instable and could thus have created the lipid divide.’

Robustness

Driessen notes that the archaeal enzymes for membrane lipid production are less specific in the reactions they catalyze than their bacterial equivalents. ‘They appear to be more “primordial”. So the evolution of enzyme specificity could have been a driver for the divide.’ There is of course one major caveat: the experiments were done in modern E. coli bacteria, which have evolved 3.5 billion years beyond the original split with the archaea.

Photo Zernike Institute for Advanced Materials, University of Groningen