Complexation and phase transfer of nucleic acids by gramicidin S

A novel interaction between gramicidin S (GrS) and nucleic acids is characterized which, like that between GrS and nucleotides, exploits both the dicationic and amphiphilic properties of the peptide. Complex formation between calf thymus DNA and GrS is demonstrated by (i) phase transfer to CHCl3 of ultrasonically irradiated DNA and (ii) inhibition of phase transfer to CHCl3 of adenosine 5'-triphosphate by either native or ultrasonically irradiated DNA. The stoichiometry of the interaction is 2:1 (DNA-P:GrS), which is consistent with a predominantly electrostatic mode of binding. The apparent affinity of GrS for DNA is considerably higher than it is for free nucleotides. The interaction of the monocationic derivative [2-N delta-acetylornithyl]gramicidin S with calf thymus DNA is considerably weaker. DNA binding by GrS provides a rationale for the lag between germination and RNA synthesis exhibited by wild-type spores of producer strains of Bacillus brevis but not by GrS-negative mutants. On the basis of these results in vitro, a protective role is proposed for GrS in the dormant spore.