Discovery of Five Classes of Bacterial Defensins: Ancestral Precursors of Defensins from Eukarya?

Defensins are present in many organisms and are divided into two evolutionary groups, termed - and -defensins. -defensins have only recently been reported in bacteria, and knowledge of these defensins is limited, with no family classification. Here, we describe the identification of 74 -defensins from bacteria and propose five classes for their classification. We also report the first NMR structure determination of a defensin, as well as its in silico expression analysis. Xanthusin-1 has a unique structure among the published defensins, which could indicate that the proposed class II peptides constitute a separate group of defensins. Xanthusin-1 gene expression was observed in casitone-based and coculture-grown media. Our results demonstrate a wider distribution of defensins outside the Eukarya domain, shedding light on the origin and distribution of defensins. The sharing of three disulfide defensins between bacteria and eukaryotes points to a possible prokaryotic origin of the CSαβ motif. Moreover, the identification of defensins in Gram-positive and Gram-negative bacteria indicates an early origin but with many gene losses during the evolutionary process, similar to findings for eukaryotic defensins.