The onset of streptothricin (ST) biosynthesis in Streptomyces rochei F20 was studied by using reverse transcription-PCR (RT-PCR) to detect transcripts of ST genes during growth in liquid medium, soil, and the rhizosphere. In situ results correlated with those obtained in vitro, illustrating the growth phase-dependent manner of ST production by F20. Maximal transcription of ST resistance (sttR) and biosynthesis (sttA) genes occurred during the transition between the exponential and stationary phases of growth, when the specific growth rate (μ) started to decline. A higher level of gene expression of sttR versus sttA was observed in all experiments. In liquid culture, maximal transcript accumulation of the sttA gene was only ca. 40% that of the sttR gene. sttA and sttR mRNAs were detected in soil containing approximately 106 CFU of growing cells g of soil−1. sttR mRNA was detected in sterile and nonsterile rhizosphere colonized with growing mycelium of F20 at 1.2 × 106 and 4.0 × 105 CFU g of soil−1, respectively. However, neither sttR nor sttA transcripts were detected by RT-PCR in the rhizoplane, which supported a lower population density of F20 than the rhizosphere.
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