Developmental significance of gramicidin S produced during sporulation in Bacillus Brevis nagano : studies associated with transcription
It has beon suggested that peptide antibiotics are involved in the transcriptional control of their producers. This problem was investigated using a wild-type strain of B. brevis nagano, the producer of gramicidin S and a GS⁻ sporulating mutant. Studies on the uptake of precursors of macromolecular synthesis in whole cells revealed that gramicidin S as well as linear gramicidin, tyrocidine and tyrothricin leave this incorporation unaffected during growth and sporulation. During germination however, C14-uracil and H3-uridine incorporation was inhibited by gramicidin S, tyrocidine and tyrothricin but not linear gramicidin. Wild-type spores are unable to outgrow in NB medium when the population density is higher than 2x10⁸ spores/ml and do not show incorporation of C14-uracil or H3-uridine. In L-alanine medium C14-uracil and H3-uridine incorporation is lower than the equivalent incorporation of mutant spores although this lower incorporation remains sensitive to inhibition by gramicidin S, tyrocidine, tyrothricin and, in this case, linear gramicidin. Studies in a semi-in vitro system showed that the antibiotics leave H3-UTP incorporation of vegetative, sporulating or germinating cells unaffected. RNA polymerase isolation from vegetative cells of the wild-type and the mutant showed transcription in vitro to be vulnerable to gramicidin S and tyrocidine inhibition. Tyrothricin was less efficient in inhibiting this transcription and linear gramicidin did not inhibit. The inhibition was found to be independent of the source or amount of RNA polymerase and the nature of template, but was found dependent on the amount of template. The antibiotics were found to complex with B. brevis DNA with an efficiency analogous to their ability to inhibit in vitro transcription (gramicidin S > tyrocidine> tyrothricin > linear gramicidin). It can be suggested that gramicidin S affects transcription only at the stage of the cell cycle that the producer is vulnerable to the antibiotic (germination/outgrowth). From this study DNA emerges as a likely candidate for antibiotic action.