A Study On The Novel Mechanism For Rapid Evolution Of Antimicrobial Peptides In Black Spotted Frog

Antimicrobial peptides (AMPs) are highly polymorphic molecules. The diversity is originated from mechanisms such as focal mutation, positive selection and balanced selection. We analyzed1050AMP cDNA sequences and found a negative correlation between the lengths of acidic propiece and mature peptide. We think such correlation underlines a novel fast evolution mechanism:movements of maturation cutting sites. In order to prove this hypothesis, we cloned114AMP cDNAs from black spotted frog (Pelophylax nigromaculata), including8cDNAs without maturation cutting site. We analyzed the rate of nonsynonymous and synonymous nucleotide replacement (dN, dS), rate of transversion (Tv) and transition (Ts), as well as phylogenetic relationships. Results demonstrated that selection pressure, despite of positive or negative selection, is not heavy, allowing fast accumulation of nonsynonymous mutations. The accumulation of nonsynonymous mutations may result in the cancellation of maturation cutting site between acidic propiece and mature peptide, and the loss of biological function. Functional silence allows AMP accumulate nonsynonymous mutations more effectively, and possibly create a novel cutting site at another place. Based on the sequence without cutting sites, we deduced chemical and physical properties for the resulting mature peptides by simulation of the creation of novel cutting sites. Results showed most potential mature paptides retain common feature of AMPs. However, such AMPs are very different from their original version. These results preliminarily explained the fast evolution mechanism and the resulting negative correlation between acidic propiece and mature peptide.