A single gene directs both production and immunity of halocin C8 in a haloarchaeal strain AS7092

Halocin C8 (HalC8) is an extremely stable and hydrophobic microhalocin with 76 amino acids, and has a wide inhibitory spectrum against the haloarchaea. It is derived from the C-terminus of a 283-amino-acid prepro-protein (ProC8), which was demonstrated by molecular cloning of the halC8 gene, and verified by the N-terminal amino acid sequencing as well as MALDI-TOF-MS analysis of the purified HalC8. The production of this halocin is controlled through both transcription regulation and protein processing: the halC8 transcripts and HalC8 activity rapidly increased to maximal levels upon transition from exponential to stationary phase. However, while halC8 transcripts remained abundant, the HalC8 processing was inhibited during stationary phase. Remarkably, agar-diffusion test revealed the unprocessed ProC8 and its 207-amino-acid N-terminal peptide (HalI), with or without the putative Tat signal sequence, were capable to block the halocin activity of HalC8 in vitro. In addition, heterologous expression of HalI in Haloarcula hispanica rendered this sensitive strain remarkable resistance to HalC8, indicating that HalI encodes the immunity property of the producer. In accordance with this immunity function, HalI and ProC8 were both found localized on the cellular membrane. Protein interaction assay revealed that HalI likely sequestrated the HalC8 activity by specific binding. To our knowledge, this is the first report on halocin immunity, and our results that a single gene encodes both peptide antibiotic and immunity protein also provide a novel immune mechanism for peptide antibiotics.