Acute transcriptional response of the honeybee peptide-antibiotics gene repertoire and required post-translational conversion of the precursor structures.

The cell-free immune repertoire of honeybees (Apis mellifera) consists of four polypeptides that are induced by bacterial infection and, through complementarity, provide broad-spectrum antibacterial defense. apidaecin is overproduced by a combination of low threshold transcriptional activation and a unique, genetically encoded amplification mechanism. In contrast, sizable experimental infections are required for induction of the normally silent hymenoptaecin, abaecin, and bee defensin genes; even so, bee defensin transcription is minimal and delayed, and only minute quantities of corresponding peptide are produced. The specific, temporal organization of the multi-component immune response in bees has therefore likely been selected to cope with infection of prevalent, plant-associated Gram-negative bacteria. Post-translational processing and modifications are substantially different for each of the four antibacterial peptides. While no similarities were observed among precursor structures of the various bee peptides, surprisingly, the signal sequences of abaecin (bee) and drosocin (Drosophila) shared unmistakable homology, possibly indicating common ancestral secretion/processing mechanisms. Finally, we report that bee defensin contains a typical disulfide-rich structure (40 amino acids) but also a unique, amphipathic, putatively amidated carboxyl-terminal tail (10 amino acids). We speculate that this structure is a "co-drug," assembled by fusing "disulfide-rich" and "alpha-helical" class peptide antibiotics, a novel concept in naturally occurring antibacterials.