Maintenance of repression control of the ilvGMEDA operon in a temperature-sensitive leucyl-transfer RNA synthetase mutant of Escherichia coli K-12 at a restrictive temperature.

The mechanism of L-leucine regulation of ilvGMEDA is thought to be by ribosome-mediated attenuation that is dependent upon the concentration of Leu-tRNA(Leu) which results from leucyl-tRNA synthetase (LeuRS) activity. The requirement for LeuRS activity in attenuation control was tested in an Escherichia coli K-12 strain containing a temperature-sensitive LeuRS and the ilvGMEDA operon with an active ilvGM. Growth of this strain at 30 degrees C followed by a shift to 37 degrees C to inactivate the LeuRS revealed that ilvGM expression decreased at the restrictive temperature whereas the downstream gene expression was slightly elevated. We suggest that ilvGM does not respond to a deattenuation signal, and that, possibly, a secondary repression/derepression mechanism exists.