Tyrocidine is a mixture of cyclic decapeptides produced by the bacteria Bacillus brevis found in soil. It can be composed of 4 different amino acid sequences, giving tyrocidine A–D (See figure 1). Tyrocidine is the major constituent of tyrothricin, which also contains gramicidin. Tyrocidine was the first commercially available antibiotic, but has been found to be toxic toward human blood and reproductive cells. The function of tyrocidine within its host B. brevis is thought to be regulation of sporulation.
Tyrocidines A, B, and C are cyclic decapeptides. The biosynthesis of tyrocidine involves three enzymes. Parts of its sequence are identical to gramicidin S.
History
In 1939, the American microbiologist René Dubos discovered the soil microbe Bacillus brevis. He observed the ability of the microbe to decompose the capsule of pneumococcus bacterium, rendering it harmless. From the soil microbe B. brevis, he isolated tyrothricin, which had a high toxicity to a large range of bacteria. Tyrothricin was later found to be a mixture of the peptides gramicidin and tyrocidine. These were observed to have toxic effects in red blood cells and reproductive cells in humans, however, if applied externally as an ointment tyrocidine could also be used as a potent antimicrobial agent. Dubos's discovery helped revive interest in research on penicillin.
Mechanism of action
Tyrocidine has a unique mode of action in which it disrupts the cell membrane function, making it a favorable target for engineering derivatives. Tyrocidine appears to perturb the lipid bilayer of a microbe's inner membrane by permeating the lipid phase of the membrane. The exact affinity and location of tyrocidine within the phospholipid bilayer is not yet known.
Biosynthesis
The biosynthesis of Tyrocidine is similar to Gramicidin S, and is achieved through the use of nonribosomal protein synthetases (NRPSs). Its biosynthesis is via an enzymatic assembly consisting of 3 peptide synthetase proteins, TycA, TycB, and TycC, which contain 10 modules. The different tyrocidine analogues (A–D) are not produced by different enzymes, but rather by an enzyme system that is capable of incorporating different amino acids of structural similarity at specified sites. The amino acid sequence is determined by the organization of the enzyme and not by any RNA template.
The tyrocidine synthetases TycA, TycB, and TycC are encoded on the tyrocidine operon. This consists of the three genes encoding for the three synthetases as well as three additional open reading frames (ORFs). These ORFs, labeled as TycD, TycE, and TycF are downstream of the three synthetase genes (see figure 2). TycD &TycE have the highest similarity to members of the ATP-binding cassette (ABC) transporter family which aid in the transport of substrates across a membrane. It has been suggested that the tandem transporters play a role in conferring resistance in the producer cell through tyrocidine secretion. TycF has been identified as a thioesterase (TE) and is similar to other TEs in bacterial operons used for encoding peptide synthetases. However, the precise function of these TEs remains unknown. The size of the peptide synthetases corresponds to the amount of activation they carry out. TycA is the smallest and activates a single amino acid from one module, TycB is intermediate in size and activates 3 amino acids with 3 modules, and TycC is the largest and activates 6 amino acids with 6 modules (See figure 3).