Tian et al. BMC Genomics 2010, 11:187 http://www.biomedcentral.com/1471-2164/11/187 Page 5 of 19 Figure 2 Clustering and structure of DLPs. A. Clustering analysis of the sequences in Figure 1 by the CLUSTAL program. Cyan and purple branches respectively represent CITDs, and nasonins and related peptides. Nasonin-2 and nasonin-6 are aligned as repeated domains; B. Structural models of DLPs from N.vitripennis. Navidefensin1-1 and navidefensin2-1 modeled ab initio by I-TASSER are shown in ribbon displayed by MolMol. Superimposition of these two structures shows an extra carboxyl-terminal domain in navidefensin1-1 structure. The model structure of nasonin-1 is compared with those of sapecin, termicin and cobatoxin. Subsequently, we evaluated antibacterial and antifun- gal activities of nasonin-1 using classical inhibition zone assays [52] and found that nasonin-1 only displayed activity against two Gram-negative bacteria at micromo- lar concentrations with a lethal concentrations (CL) of 1.52 μM for Stenotrophomonus sp. LZ-1 and of 15.4 μM for Escherichia coli ATCC 25922 (Figure 4C). No effect was observed on Gram-positive bacteria and fungi. This is the first example describing a DLP mainly active on Gram-negative bacteria. Recombinant navidefensin2-2 was recently reported to be active on Gram-positive bacteria [25]. Whether such remarkable difference in target selectivity is associated with the change of n-loop length in these two Nasonia defensins needs further investigation. AMPs rich in specific amino acids AMPs rich in specific amino acids (e.g. glycine, proline, histidine, tryptophan or arginine) represent an additional class of AMPs acting on diverse microorganisms (e.g. bacteria, fungi, and virus) [53,54]. The majority of these molecules are unstructured and they often inhibit microbial growth by entering into cells and interacting with proteins involved in key metabolic processes [15]. Eighteen such AMPs have been identified here, includ- ing two known AMP families (abaecin and hymenoptae- cin) [55,56]. (1) Abaecin Abaecin is an inducible Pro-rich antibacter- ial peptide, firstly identified from the hemolymph of A. mellifera after bacterial challenge, and known to be regulated by the immune deficiency (IMD) pathway [55,57]. They can act on both Gram-positive and Gram- negative bacteria with a moderate inhibitory concentra- tion when compared to apidaecin which was also from A. mellifera [55]. Subsequently, several abaecin-like genes were identified from the bee A. cerana and Bom- bus ignites, and the wasp Pteromalus puparum [58,59]. Searching for the N. vitripennis genome identified three genes encoding protein precursors with clear sequence similarity to abaecins in their amino-terminal part of mature peptides. We named these abaecin-like genes nabaecin-1 to -3. Analysis of these precursor sequences allowed recognizing a dibasic cleavage site (RR) for putative post-translational processing by which an amino-terminal part (called N-terminal abaecin unit, NtAU) corresponding to the bee abaecin will be released (Figure 5). The NtAU of nabaecin-3 shares the highest