Fig. 5

Proposed interaction between our reconstructed CPR species and their potential THF-auxotrophic hosts. A The conceptual model infers the interactions between CPR species and their potential THF-auxotrophic microbial hosts. The proposed exchange of THF and other essential cellular components between CPR and their auxotrophic bacterial hosts were shown in dashed lines with arrows. B The structural arrangement of the CPR DHFRs encoded by the detected folA genes was modeled using Alphafold2 [63]. The folate-binding sites were highlighted in red. C The growth curves of the E. coli MG1655 wild type, the ΔfolA mutant containing the empty vector, and the complementary ΔfolA mutants containing the plasmid pBBR1MCS-2 harboring one of the folA genes from CPR bacteria. The error bars indicated the standard deviation of three replicates. ΔfolA::pBBR1MCS-2: the empty plasmid pBBR1MCS-2 was transformed into strain ΔfolA; ΔfolA::pBBR1MCS-2-folA_31/34/36: the recombinant plasmid pBBR1MCS-2 inserted with individual folA gene from the CPR MAGs (bin31, bin34 and bin36) was transformed into strain ΔfolA (Table S2). The complementary strains were incubated in an LB medium without thymidine at 37 °C