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The str operon of Streptomyces ramocissimus contains the genes for ribosomal proteins S12 (rpsL) and S7 (rpsG) and for the polypeptide chain elongation factors G (EF-G) (fus) and Tu (EF-Tu) (tuf). This kirromycin producer contains three tuf or tuf-like genes; tuf1 encodes the regular EF-Tu and is located immediately downstream of fus. In vivo and in vitro transcription analysis revealed a transcription start site directly upstream of S. ramocissimus tuf1, in addition to the operon promoter rpsLp. Transcription from these promoters appeared to be growth phase dependent, diminishing drastically upon entry into stationary phase and at the onset of production of the EF-Tu-targeted antibiotic kirromycin. In surface-grown cultures, a second round of tuf1 transcription, coinciding with aerial mycelium formation and kirromycin production, was observed. The tuf1-specific promoter (tuf1p) was located in the intercistronic region between fus and tuf1 by high-resolution S1 mapping, in vitro transcription, and in vivo promoter probing. During logarithmic growth, the tuf1p and rpsLp transcripts are present at comparable levels. In contrast to Escherichia coli, which has two almost identical tuf genes, the gram-positive S. ramocissimus contains only tuf1 for its regular EF-Tu. High levels of EF-Tu may therefore be achieved by the compensatory activity of tuf1p.

Growth phase-dependent transcription of the Streptomyces ramocissimus tuf1 gene occurs from two promoters