AMINOACYL-TRANSFER-RNA SYNTHETASE GENE-REGULATION IN BACILLUS-SUBTILIS - INDUCTION, REPRESSION AND GROWTH-RATE REGULATION

The thrS gene in Bacillus subtilis is specifically induced by starvati on for threonine and is, in addition, autorepressed by the overproduct ion of its own gene product, the threonyl-tRNA synthetase. Both method s of regulation employ an antitermination mechanism at a factor-indepe ndent transcription terminator that occurs just upstream of the start codon. The effector of the induction mechanism is thought to be the un charged tRNA(Thr), which has been proposed to base pair in two places with the leader mRNA to induce antitermination. Here we show that the autoregulation by synthetase overproduction is likely to utilize a mec hanism similar to that characterized for induction by amino acid starv ation, that is by altering the levels of tRNA charging in the cell. We also demonstrate that the base pairing interaction at the two propose d contact points between the tRNA and the leader are necessary but not always sufficient for either form of regulation. Finally, we present evidence that the thrS gene is expressed in direct proportion to the g rowth rate. This method of regulation is also at the level of antiterm ination but is independent of the interaction of the tRNA with the lea der region.