PROCESSING OF THE LEADER MESSENGER-RNA PLAYS A MAJOR ROLE IN THE INDUCTION OF THRS EXPRESSION FOLLOWING THREONINE STARVATION IN BACILLUS-SUBTILIS

The threonyl-tRNA synthetase gene, thrS, is a member of a family of Gr am-positive genes that are induced following starvation for the corres ponding amino acid by a transcriptional antitermination mechanism invo lving the cognate uncharged tRNA. Here we show that an additional leve l of complexity exists in the control of the thrS gene with the mappin g of an mRNA professing site just upstream of the transcription termin ator in the thrS leader region. The processed RNA is significantly mor e stable than the full-length transcript. Under nonstarvation conditio ns, or following starvation for an amino acid other than threonine, th e full-length thrS mRNA is more abundant than the processed transcript . However, following starvation for threonine, the thrS mRNA exists pr imarily in its cleaved form. This can partly be attributed to an incre ased processing efficiency following threonine starvation, and partly to a further, nonspecific increase in the stability of the processed t ranscript under starvation conditions. The increased stability of the processed RNA contributes significantly to the levels of functional RN A observed under threonine starvation conditions, previously attribute d solely to antitermination. Finally, we show that processing is likel y to occur upstream of the terminator in the leader regions of at leas t four other genes of this family, suggesting a widespread conservatio n of this phenomenon in their control.