The microviridins are a group of ribosomally synthesized and subsequently posttranslationally modified peptides. The structural modifications introduced during maturation are the formation of two intramolecular esters and one amide bond accompanied by dehydration. The two ester bonds are introduced by one GRASP-like ligase (ATP-dependent carboxylate-amine/thiol ligase) (Galperin & Koonin, 1997) and the amide bond is formed by a second such enzyme, which shows strong homology to the ligase introducing the ester bonds. Action of these two enzymes gives microviridins an overall tricyclic topography. Further maturation of the peptide is achieved by leader peptide cleavage and N-terminal acetylation. Members of this group have been isolated and characterized by spectroscopic methods exclusively from the cyanobacteria, specifically the genera Microcystis, Nostoc, and Planktothrix (Oscillatoria). Expression of two genes encoding GRASP-like ATP-binding proteins has made it possible to study the cyclization reaction in vitro and to define the minimal sequence requirements for cross-linking in the C-terminal region comprising the structural peptide. Heterologous expression of the microviridin gene cluster of Microcystis in Escherichia coli and analysis of the cell mass of the heterologous host has allowed the analysis of motifs in the leader peptide important for posttranslational modification.
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