ApnI, a Transmembrane Protein Responsible for Subtilomycin Immunity, Unveils a Novel Model for Lantibiotic Immunity

Subtilomycin was detected from the plant endophytic strainBacillus subtilis BSn5 and was first reported fromB. subtilis strain MMA7. In this study, a gene cluster that has been proposed to be related to subtilomycin biosynthesis was isolated from the BSn5 genome and was experimentally validated by gene inactivation and heterologous expression. Comparison of the subtilomycin gene cluster with other verified related lantibiotic gene clusters revealed a particular organization of the genesapnI andapnT downstream ofapnAPBC , which may be involved in subtilomycin immunity. Through analysis of expression of theapnI and/orapnT genes in the subtilomycin-sensitive strain CU1065 and inactivation ofapnI andapnT in the producer strain BSn5, we showed that the single geneapnI , encoding a putative transmembrane protein, was responsible for subtilomycin immunity. To our knowledge, evidence for lantibiotic immunity that is solely dependent on a transmembrane protein is quite rare. Further bioinformatic analysis revealed the abundant presence of ApnI-like proteins that may be responsible for lantibiotic immunity inBacillus andPaenibacillus . We cloned thepaeI gene, encoding one such ApnI-like protein, into CU1065 and showed that it confers resistance to paenibacillin. However, no cross-resistance was detected between ApnI and PaeI, even though subtilomycin and paenibacillin share similar structures, suggesting that the protection provided by ApnI/ApnI-like proteins involves a specific-sequence recognition mechanism. Peptide release/binding assays indicated that the recombinantB. subtilis expressingapnI interacted with subtilomycin. Thus, ApnI represents a novel model for lantibiotic immunity that appears to be common.