Biological function of gramicidin: studies on gramicidin-negative mutants

By the use of a rapid radioautographic screening procedure, two mutants of Bacillus brevis ATCC 8185 that have lost the ability to produce gramicidin have been isolated. These mutants produced normal levels of tyrocidine and sporulated at the same frequency as the parent strain. Their spores, however, were more heat-sensitive and had a reduced dipicolinic acid content. Gramicidin-producing revertants occurred at a relatively high frequency among the survivors of prolonged heat treatment and had also regained the ability to produce heat-resistant spores. A normal sport phenotype could also be restored by the addition of gramicidin to cultures of the mutant strain at the end of the exponential growth. On the other hand, the addition of dipicolinic acid could not cure the spore defect. These results provide strong evidence that the inability to produce gramicidin is directly responsible for the observed spore defects. Indeed, they unambiguously demonstrate a function of a peptide antibiotic in bacterial sporulation. The possibility that this function consists of the regulation of transcription during the transition from growth to sporulation is discussed.