GROWTH PHASE-DEPENDENT TRANSCRIPTION OF THE STREPTOMYCES-RAMOCISSIMUSTUF1 GENE OCCURS FROM 2 PROMOTERS

The sfr operon of Streptomyces ramocissimus contains the genes for rib osomal proteins S12 (rpsL) and S7 (rpsG) and for the polypeptide chain elongation factors G (EF-G) (fus) and Tu (EF-Tu) (tuf). This kirromyc in producer contains three tuf or tuf-like genes; tuf1 encodes the reg ular EF-Tu and is located immediately downstream of fus. In vivo and i n vitro transcription analysis revealed a transcription start site dir ectly upstream of S. ramocissimus tuf1, in addition to the operon prom oter rpsLp. Transcription from these promoters appeared to be growth p hase dependent, diminishing drastically upon entry into stationary pha se and at the onset of production of the EF-Tu-targeted antibiotic kir romycin. In surface-grown cultures, a second round of tuf1 transcripti on, coinciding with aerial mycelium formation and kirromycin productio n, was observed. The tuf1-specific promoter (tuf1p) was located in the intercistronic region between fus and tuf1 by high-resolution S1 mapp ing, in vitro transcription, and in vivo promoter probing. During loga rithmic growth, the tuf1p and rpsLp transcripts are present at compara ble levels. In contrast to Escherichia coli, which has two almost iden tical 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.