Further molecular analysis of a bacteriocin produced by the sweet potato pathogen Streptomyces ipomoeae that shows interstrain inhibition

Soil rot is a destructive disease of sweet potato caused by the bacterium Streptomyces ipomoeae. Strains of S. ipomoeae have been divided into three groups based on their antagonistic characteristics during co-cultivation on agar media. Group I strains are not antagonistic to group II or group III strains, group II strains inhibit both group I and III members, and group III strains inhibit group I and II strains. The group III inhibitor, ipomicin, consists of a 10-kDal secreted protein with bacteriocin properties, which may prove useful as a biocontrol agent. To investigate ipomicin production further, we performed a kinetic analysis of ipomicin production during liquid growth of a representative group III strain. Ipomicin activity was found to increase by tenfold upon shift of the culture into stationary phase. This increase in activity appeared to correlate with an increasing amount of the 10-kDal mature form of ipomicin in culture supernatants; however, a concomitant increase in transcription of the ipomicin gene ipoA was not observed, thereby indicating that regulation appears to be post-transcriptional. Interestingly, a rare TTA codon, which is the target of bldA tRNA-mediated translational control in other Streptomyces spp., is found in the signal sequence portion of the ipoA gene and suggests a possible regulatory mechanism for ipomicin production here.