Improvement of the Genome Editing Tools Based on 5FC/5FU Counter Selection in Clostridium acetobutylicum

Several genetic tools have been developed for Clostridium acetobutylicum utilizing 5-fluorouracil (5FU) or 5-fluorocytosine (5FC) resistance as a selection method. A method based on the integration, by single crossing-over, of a suicide plasmid (pCat-upp) followed by selection for the second crossing-over using a counter-selectable marker (the upp gene and 5FU resistance) was recently developed for genome editing in C. acetobutylicum. This method allows the modification of the genome without any marker or scar left in a strain of C. acetobutylicum that is ∆upp. Unfortunately, 5FU has strong mutagenic properties inducing mutations in the strain’s genome. After numerous applications of the pCat-upp/5FU system for genome modification in C. acetobutylicum, the bacteria became completely resistant to 5FU in the presence of the upp gene, resulting in failure in the selection for the second crossing-over. It was found that the potential repressor of the pyrimidine operon, PyrR, was mutated at position A115, leading to the 5FU resistance of the strain. We developed two plasmids, one overexpressing the native pyrR gene and a suicide plasmid carrying a non-mutated and optimized pyrR gene (pyrR*) and upp, which allowed us to restore the 5FU sensitivity of the strain. We also improved the pCat-upp/5FU system by reducing the concentration of 5FU from 1mM to 5 µM using a define synthetic medium.