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Antibacterial action of gramicidin S and tyrocidines in relation to active transport, in vitro transcription, and spore outgrowth

The cyclopeptide antibiotic gramicidin S or tyrocidine in concentrations of 2 to 4 mumol/mg of membrane protein inhibited the active transport of [3H]alanine and [3H]uridine in membrane vesicles isolated from Bacillus brevis and Bacillus subtilis. We used one analog of gramicidin S and two of tyrocidine A to study the relationship between peptide structure and antibacterial action as seen in inhibiting active transport and in vitro transcription and in delaying spore outgrowth. The data showed that [Ser2,2']-gramicidin S, in which the two ornithine residues were replaced by two serines, was at least 50 times less active antibacterially and gave a low response in transport inhibition and delay of spore outgrowth compared with the natural peptide. The antibacterial activity of [Val6]-tyrocidine A was twice lower than that of tyrocidine A, and it also showed a considerable reduction in transport and transcription inhibition. [Orn7]-tyrocidine A containing two ornithine residues in positions corresponding to those in gramicidin S was almost inactive in all functions tested. The correlation between peptide structure and activity is discussed.