An interaction between gramicidin and the sigma subunit of RNA polymerase.

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Abstract
Gramicidin, a peptide antibiotic produced by Bacillus brevis, inhibits initiation of transcription by RNA polymerase (nucleosidetriphosphate:RNA nucleotidyltransferase, EC 2.7.7.6). The presence of gramicidin causes an increase in the rate of cleavage of the .sigma. subunit of Escherichia coli RNA polymerase by trypsin, although it does not alter the cleavage rate of any of the core subunits. Whereas isolated .sigma. is cleaved much faster than is .sigma. in holoenzyme, gramicidin substantially decreases the trypsin cleavage rate of isolated .sigma.. Inhibition of RNA polymerase activity by gramicidin is consistent with a .sigma.-specific effect; the antibiotic is a strong inhibitor of transcription of T7 phage DNA, which requires .sigma. for activity, but it has little effect on transcription of .sigma.-independent templates, such as poly(dA-dT).cntdot.poly(dA-dT) and calf thymus DNA. These results are discussed in light of the hypothesized role for gramicidin in the initiation of sporulation of B. brevis.