Transcription of the ileS operon in the archaeon Methanobacterium thermoautotrophicum Marburg.

In the thermophilic archaeon Methanobacterium thermoautotrophicum Marburg, the structural gene for isoleucyl-tRNA synthetase (ileS) is flanked upstream by orf401 and downstream by purL. orf401 encodes a 43.5-kDa protein with an unknown function. Northern (RNA) hybridization and S1 nuclease protection experiments showed that the orf401, ileS, and purL genes are cotranscribed from an archael consensus promoter in front of orf401. The corresponding transcript was about eightfold increased in cells that had been exposed to pseudomonic acid A, a specific inhibitor of isoleucyl-tRNA synthetase. Growth inhibition by puromycin, tryptophan starvation, or starvation for hydrogen did not affect the level of this transcript. The level of a trpE transcript, however, was drastically elevated upon tryptophan starvation, while inhibition by pseudomonic acid A had no effect on the level of this transcript. Expression of ileS thus appears to be controlled by a regulatory mechanism which specifically responds to the availability of isoleucyl-tRNA. Extensive decay of the orf401-ileS-purL message was observed. Degradation occurred, presumably by endonucleolytic cleavage, within the orf401 region.