Expression and activity of cyclic and linear analogues of esculentin-1, an anti-microbial peptide from amphibian skin.

Esculentin-1 is a potent anti-microbial peptide present in minute amounts in skin secretions of Rana esculenta. It contains 46 amino-acid residues and a C-terminal disulfide bridge. We have explored the possibility of producing analogues of this peptide by recombinant expression in Escherichia coli of a fusion protein which is sequestered in inclusion bodies. The peptide of interest has been inserted at the N-terminus of the protein, from which it can be released by cyanogen bromide cleavage. The anti-microbial activities of the recombinant peptide as well as that of a mutant linear form devoid of the disulfide bridge are presented. The recombinant analogues retain the biological activity of the natural peptide, as tested with an inhibition zone assay against a variety of microorganisms. However, experiments on the rate of bacterial killing show that gram-negative bacteria are more sensitive to the peptides than the gram-positive bacterium, the effect of the cyclic peptide being in all cases faster than that of the linear molecule. Moreover, the activity against gram-negative bacteria for both peptides is not affected by salts, whereas the activity against Staphylococcus aureus is lost at high salt concentration.