threonine residue found in cyclothiazomycin A and a serine residue in place of the third threonine residue, resulting in a dehydroalanine instead of the dehydrohomoalanine observed in cyclothiazomycin A. Biosynthesis Cyclothiazomycin is part of a class of natural products that are ribosomally synthesized and post-translationally modified peptides (RiPPs). As such, cyclothiazomycin begins as a peptide that is synthesized in the bacterial ribosome. A series of chemical steps by biosynthetic enzymes transform the original peptide into the final (mature) natural product. The gene encoding for cyclothiazomycin begins with a short open reading frame (ORF) ctmA (cltA). CtmD (cltB) encodes a "fused" TOMM cyclodehydratase believed to take part in the formation of thiazolines. The enzyme encoded by the ctmB (cltC) gene is believed to catalyze the dehydrogenation of thiazolines to thiazoles. CtmE and ctmF (ctlE and cltF) each encode a split lanthipeptide dehydratase which dehydrates serine and threonine to dehydroalanine and dehydrobutyrine. CtmG (cltG) is believed to aide in the production of the central pyridine. CtmI (cltD) encodes a ThiF-like protein. CtmH (cltH) is a LuxR-type regulatory gene. CtmJK are not present in the cycloythiazomycin A and C clusters; there is no known function. The cltN gene is believed to encode an enzyme that is involved in the formation of the tertiary thioether, however this has not been proven and cltN is not regulated by ctmH.CtmA, ctmD, ctmF, and ctmG deletion leads to the disappearance of cyclothiazomycin A. CtmH is a LuxR-type regulatory gene that was shown to regulate ctmA-ctmL. Bioactivity Each analogue of cyclothiazomycin is believed to interact with at least one biological target. Cyclothiazomycin A has been shown to inhibit blood plasma renin leading to a decrease blood pressure; it also exhibits weak anti-fungal activity. Cyclothiazomycin B shows inhibitory activity against RNA polymerase, and it is believed to act by reducing ribosome dependent GTPase. Cyclothiazomycin B also exhibits anti-fungal activity by binding to the chitin of cell walls, thus producing a fragile cell wall. Recently discovered cyclothiazomycin C has an unknown biological role, but cyclothiazomycins B and C both exhibit antibacterial activity against Gram-positive bacteria, including Bacillus anthracis. References Thiopeptides