Proposed microviridin and oscillatorin biosynthetic pathways. Orange arrows, with protein name and predicted function above, illustrate the genes size and transcription direction. a) MdnA is proposed as microviridin precursor. MdnCB are predicted to ligate the ring structures, MdnD is responsible for acetylation. A microviridin with unknown side chain was predicted based on the precursor sequence (MdnA) and gene similarities to the Microcystis microviridin gene clusters . b) The alignment of sequences with high similarities to Microcystis MdnA shows two highly conserved regions in the leader peptide (green dashed line) and core peptide (red dashed line). The leader-core transition is unknown. c) oscA is proposed as the oscillatorin precursor encoding 619 amino acids, includes eight of the 10 amino acids in oscillatorin (bold text). The tryptophan is suggested as a precursor to oscillatoric acid (Osc). Based on in silico analyses of the gene cluster, the oscillatorin precursor is likely to be cleaved by peptidase S33. Modification of tryptophan and addition of valine is carried out by unknown enzymes. A putative cyclo-ligase is may be involved in cyclization.