Antimicrobial Peptides Derived from Ascidians and Associated Cyanobacteria

Ascidians belonging to Phylum Chordata are the most largest and diverse of the Sub-phylum Tunicata (Urochordata). Marine ascidians are one of the richest sources of bioactive peptides. These bioactive peptides from marine ascidians are confined to various types of structures such as cyclic peptides, acyclic peptides (depsipeptides), linear helical peptides with abundance of one amino acid (proline, trytophane, histidine), peptides forming hairpin like beta sheets or α-helical/β-sheet mixed structures stabilized by intra molecular disulfide bonding. Cyanobactins are fabricated through the proteolytic cleavage and cyclization of precursor peptides coupled with further posttranslational modifications such as hydroxylation, glycosylation, heterocyclization, oxidation, or prenylation of amino acids. Ascidians are known to be a rich source of bioactive alkaloids. β-carbolines form a large group of tryptophan derived antibiotics. Pyridoacridines from ascidians are tetra- or penta- cyclic aromatic alkaloids with broad range of bioactivities. Didemnidines derived from ascidian symbiotic microbes are inhibitors of phospholipase A2 and induce cell apoptosis. Meridianins are indulged in inhibiting various protein kinases such as, cyclindependent kinases, glycogen synthase kinase-3, cyclic nucleotide dependent kinases, casein kinase, and also implicate their activity of interfering with topoisomerase, altering the mitochondrial membrane potential and binding to the DNA minor groove to inhibit transcriptional activation. Most of these bioactive compounds from ascidians are already in different phases of the clinical and pre-clinical trials. They can be used for their nutraceutical values because of their antineoplastic, antihypertensive, antioxidant, antimicrobial, cytotoxic, antibacterial, antifungal, insecticidal, anti-HIV and anti-parasitic, anti-malarial, anti-trypanosomal, anti-cancer etc. This chapter mostly deals with antibacterial compounds from ascidian and their associate symbiotic cyanobacteria.