A bacterium that makes biofuels also needs to tolerate living in a solvent soup. The authors of a study in mBio this week amped up promoters for various heat shock proteins (HSPs) in E. coli and created a system they say could be used to create tailor-made organisms for producing ethanol, n-butanol, and other products bacteria usually try to avoid.
When you hear “solvent tolerance”, heat shock proteins might not immediately come to mind, but it makes sense: both high temperatures and solvents result in damaged or mis-folded proteins, problems that HSPs can remedy. Other studies have recognized that HSPs are up-regulated in solvent-tolerant bacteria, and a couple studies introduced heterologous HSPs into E. coli to create greater solvent tolerance. (In case you needed it, HSPs provide more proof that names can be misleading – they might just as well be called SSPs: Solvent shock proteins. Their invocation as a response to heat was just discovered first.)
"Tuning" overexpression to suit the needs of the manufacturer
Zingaro et al. scrutinized the HSP response of E. coli in solvent solutions and identified the most beneficial HSP genes for tweaking: GrpE and ClpB and two proteins in the GroESL system. By using the separate promoters for these three entities, and by boosting the plasmid copy number for GrpE and ClpB, which are carried on plasmids, Zingaro et al. were able to create strains of E. coli pre-prepped for exposure to solvents.
The results are promising, they say. By counting the cells that survived the solvent onslaught, they showed tuned coexpression of these HSP genes created E. coli that was up to 1,130% more tolerant of ethanol (GroESL + ClpB), 390% more tolerant of n-butanol (GrpE, GroESL, and ClpB on a single plasmid), and 78% more tolerant of 1,2,4-butanetriol (again, GrpE, GroESL, and ClpB on a single plasmid).
According to the authors, tuning HSPs, finding just the right combination of overexpressed genes like this, could be used to build custom-engineered systems for manufacturing biofuels and other solvents. This particular system would need to be tweaked to make it practical for use in manufacturing, they write, but it lays some of the necessary groundwork.