Fig. 1

Phylogeny of MCR-4 and its variants. a An unrooted radial phylogram of MCR-4 and its homologs Amino acid sequences of close homologs of MCR-4, in combination with those from MCR-1/2 homologs have been included in the analysis. Three distinct phylogenetic groups refer to MCR-4 variants (in orange), MCR-3 variants (in yellow), and MCR-1/2 variants (in pink), respectively. In particular, a chromosomally-encoded colistin resistance determinant, Neisseria eptA is distinct member. Z1140 is an experimentally-verified non-functional PE transferase and acts as an internal reference in this phylogeny^9,10. b Rooted phylogenetic tree of MCR-4 and its close homologs. Two distinct closely-clustered subclades have been indicated, including MCR-4 variants (highlighted in red with orange background) and MCR-4 homologs from Shewanella species (in a light blue background). The tree has been rooted with Z1140, an experimentally-verified non-functional PE transferase^9,10. A protein sequence-based phylogenetic analysis of MCR-4 was constructed using the maximum likelihood method. Sequences were aligned using MUSCLE and phylogenetic trees here have been inferred using the LG model. A discrete gamma distribution was used to model evolutionary rate differences among sites with some evolutionarily invariable sites. The percentages of replicate trees in which the associated taxa are clustered in the bootstrap test (1000 replicates) is shown next to the branches. The tree is drawn to scale, with branch lengths measured in the number of substitutions per site. Protein accession numbers have been indicated in the figure. *mcr-4.3 is renamed from a duplicated mcr-4.2, and then found to be an inactive version³⁴. The scale of bar in the phylogenetic tree is 0.20