Characterization of NMCR-2, a new non-mobile colistin resistance enzyme: implications for an MCR-8 ancestor.

MCR-4 and MCR-8 are two recently identified members of an ongoing MCR family of colistin resistance. Although that aquatic reservoir for MCR-4 is proposed, the origin and mechanism of MCR-8 is poorly understood. Here we report a previously unrecognized non-mobile colistin resistance enzyme, termed NMCR-2, originating from the plant pathogen Kosakonia pseudosacchari. NMCR-2 (551aa) gives 67.3% identity to MCR-8 (565aa). NMCR-2 is placed as a progenitor/ancestor for MCR-8 in phylogeny of MCR members. Genetic study reveals that nmcr-2 is comparable to mcr-8 in the ability of producing phenotypic colistin resistance. Biochemical analyses determine that these two enzymes catalyse the transfer of PEA from the donor PE lipid substrate to the recipient lipid A molecule by a putative 'ping-pong' trade-off. Further experiment of protein engineering demonstrates that the two motifs (TM region and catalytic domain) of NMCR-2 are functionally exchangeable with that of MCR-8, rather than MCR-1. Physiological impacts of nmcr-2 and/or mcr-8 are detected in Escherichia coli, featuring with fitness cost. Evidently, the action and mechanism of NMCR-2 is analogous to that of MCR-8. Therefore, our finding underlines that NMCR-2 might be a possible progenitor of MCR-8.