EXPRESSION OF BOTH BACILLUS-SUBTILIS THREONYL-TRANSFER-RNA SYNTHETASEGENES IS AUTOGENOUSLY REGULATED

The ''housekeeping'' threonyl-tRNA synthetase gene (thrS) of Bacillus subtilis is shown to be transcribed in vivo and in vitro from a single promoter. In vitro, 85% of all messages transcribed from the thrS pro moter are terminated at a strong factor-independent terminator localiz ed upstream of the thrS Shine-Dalgarno sequence, within the 305-nucleo tide-long leader region. Overexpression of thrS represses transcriptio nal and translational thrS-lacZ fusions to a similar extent, suggestin g that thrS is autoregulated at the transcriptional level. We show tha t autogenous control does not act at the level of transcription initia tion but involves antitermination of the transcription mechanism. thrZ , the second threonyl-tRNA synthetase gene, is also autogenously regul ated. However, the ability of the ThrS synthetase to repress thrS as w ell as thrZ expression is much greater than that of the ThrZ synthetas e.