Deletion of atypical type II restriction genes in Clostridium cellulovorans using a Cas9-based gene editing system

The anaerobic bacterium Clostridium cellulovorans is a promising candidate for the sustainable production of biofuels and platform chemicals due to its cellulolytic properties. However, the genomic engineering of the species is hampered because of its poor genetic accessibility and the lack of genetic tools. To overcome this limitation, a protocol for triparental conjugation was established that enables the reliable transfer of vectors for markerless chromosomal modification into C. cellulovorans. The availability of reporter genes is another requirement for strain engineering and biotechnological applications. In this work, the oxygen-free fluorescence absorption-shift tag (FAST) system was used to characterize promoter strength in C. cellulovorans. Selected promoters were used to establish a CRISPR/Cas system for markerless chromosomal modifications. For stringent control of expression of Cas9, a theophylline-dependent riboswitch was used, and additionally, the anti-CRISPR protein AcrIIA4 was used to reduce the basal activity of the Cas9 in the off-state of the riboswitch. Finally, the newly established CRISPR/Cas system was used for the markerless deletion of the genes encoding two restriction endonucleases of a type II restriction-modification (RS) system from the chromosome of C. cellulovorans. In comparison to the WT, the conjugation efficiency when using the deletion mutant as the recipient strain was improved by about one order of magnitude, without the need for prior C. cellulovorans-specific in vivo methylation of the conjugative plasmid in the E. coli donor strain. KEY POINTS: • Quantification of heterologous promoters enables rational choice for genetic engineering. • CRISPR/Cas with riboswitch and anti-CRISPR allows efficient gene deletion in C. cellulovorans. • Conjugation protocol and type II REase deletion enhance genetic accessibility.