Discovery of "heat shock metabolites" produced by thermotolerant actinomycetes in high-temperature culture

In actinomycetes, many secondary metabolite biosynthetic genes are not expressed under typical laboratory culture conditions and various efforts have been made to activate these dormant genes. In this study, we focused on high-temperature culture. First, we examined the thermotolerance of 3160 actinomycete strains from our laboratory culture collection and selected 57 thermotolerant actinomycetes that grew well at 45 degrees C. These 57 thermotolerant actinomycetes were cultured for 5 days in liquid medium at both 30 degrees C and 45 degrees C. Culture broths were extracted with 1-butanol, and each extract was subjected to LC/MS analysis. The metabolic profiles of each strain were compared between the 30 degrees C and 45 degrees C cultures. We found that almost half of these thermotolerant actinomycetes produced secondary metabolites that were detected only in the 45 degrees C culture. This result suggests that high-temperature culture induces the production of dormant secondary metabolites. These compounds were named "heat shock metabolites (HSMs)." To examine HSM production in more detail, 18 strains were selected at random from the initial 57 strains and cultivated in six different media at 30 degrees C and 45 degrees C; as before, metabolic profiles of each strain in each medium were compared between the 30 degrees C and 45 degrees C cultures. From this analysis, we found a total of 131 HSMs. We identified several angucycline-related compounds as HSMs from two thermotolerant Streptomyces species. Furthermore, we discovered a new compound, murecholamide, as an HSM from thermotolerant Streptomyces sp. AY2. We propose that high-temperature culture of actinomycetes is a convenient method for activating dormant secondary metabolite biosynthetic genes.