**5.2 Identification and characterization of cyanobactins**

Cyanobactins are a family of small, cyclic peptides produced by cyanobacteria and consist of N-to-C macro-cylization of a 6–20 amino acid chain. They are generally assembled through the cleavage and modification of short precursor proteins. Many of these peptides show antimalarial or antitumor activity [97]. It is speculated that close to 30% of all cyanobacterial strains contain genes corresponding to synthesis of cyanobactins [98, 99]. It is also speculated that, bacterial diversity other than cyanobacteria may also have harbor the gene(s) and gene cluster(s) for synthesis of cyanobactins [98]. However, access to such cyanobactins gene cluster(s) is limited due to the non- cultivability of the vast microbial majority. A few metagenomic studies have reported cloning and heterologous expression of biosynthetic gene clusters for the cyanobactins. In one such example study, the gene cluster for 'patellamide' was cloned and heterologously expressed from metagenomic libraries of uncultured cyanobacterial symbionts associated with marine sponge [100, 101]. In other studies, the structural diversity of diversity was enriched with subtle changes in the gene encoding for precursor peptide and employed it in combination with multiple strategies e.g. (i) orthogonal loading of unnatural amino acids; (ii) mutagenesis of precursor peptide; (iii) generation of a library of hybrid cyanobactins [90].
