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E. coli is firmly established as a workhorse in every laboratory. It willingly takes up foreign DNA and expresses it at high levels. When it comes to giving up the fruit of its labor, however, the workhorse becomes less cooperative, and purification of exogenous proteins from endogenous E. coli proteins can be tedious. Masayori Inouye and his colleagues at Robert Wood Johnson Medical School, found a way to change that by enzymatically persuading E. coli to get rid of all its endogeous mRNA and to focus entirely on the production of the foreign protein (Suzuki et al., 2005).

In this method, Inouye is exploiting a system E. coli developed to help it through times of stress. The bacterium continuously expresses the suicide operon, encoding the toxin mazF and the antitoxin mazE; in times of abundance these two proteins form a complex and the toxin is neutralized. If, however, E. coli encounters stress, protein production is halted and mazE, the less stable of the two, is degraded. This leaves the toxin to recognize the specific sequence ACA on any cellular mRNA and destroy it, pushing the cell into a dormant state during which it does not grow but can wait out the crisis.

Inouye's team, less interested in helping E. coli through the tough times, but more intrigued by the implications of destroying all cellular mRNA, overexpressed mazF together with a gene of interest in which all ACA sequences had been replaced by silent mutations. This ACA-less mRNA was translated very efficiently, while all endogenous mRNA had disappeared, resulting in E. coli cells filled almost exclusively with the protein of interest.

Inouye emphasized two applications of his system. The first is the study of protein structure in their natural environment by NMR. He explains, “We don't even want to break open the cell, but [instead] try to discover the protein structure inside the cell.” The second involves finding new jobs for E. coli. “We think this system can be used to convert a living cell into a bioreactor,” says Inouye. “It won't be restricted to proteins, it could be producing small biochemical molecules.” Expression of mazF would ensure that all the endogeous mRNA is destroyed and the cell could channel all its energy to making, for example, a specific amino acid.

Despite all these intriguing possibilities, E. coli, being a bacterium, has difficulties correctly folding and modifying eukaryotic proteins. Therefore, Inouye is now working on adapting his method of single protein production also to yeast and mammalian cells. It remains to be seen whether these will be the same willing workhorses that E.coli has been.