The Lenski Experiments
In the Lenski experiment the E.Coli cells grown in the presence of oxygen gained the ability to transport citrate across the membrane. The allowed them to utilize citrate as a carbon source and provided a selective advantage in the conditions of the experiment.
In the absence of oxygen with another suitable source of energy the E.coli cells are able to transport and use citrate.
The mutation(s) that occurred in the Lenski experiment therefore allowed (unusually) the transport of citrate across the membrane in the presence of oxygen.
As far as I am aware the detailed molecular story of what happened in this particular case has yet to be unravelled. It seems that at least two different mutations must occur for this ability to be conferred.
As far as I am aware no new proteins are involved. The most likely explanations are a loss of the usual control of the anaerobic citrate transport system or a mutation in a protein that transports a similar molecule
What is significant from these results is that even for this small modification in an existing protein 31,500 generations were required with a population size of about 5 million. This was in the presence of the heaviest selection pressure possible.
Behe argues that this work is consistent with his arguments regarding the limit of evolution.
Behe’s discussion of the Lenski work.
A recent Scientific American article on the Lenski experiment. (HT to Psilo)