Four Escherichia coli operons, the leuV operon which encodes
tRNA(1Leu), the leuX operon which encodes
tRNA(6Leu), the metT operon which encodes
tRNA(3Leu), and the argT operon which encodes
tRNA(1Leu), were examined for the stringent response induced by
serine hydroxamate and for growth rate-dependent regulation. In nuclease protection assays, the leuV operon displayed the stringent response in response to
leucine starvation, analog inhibition, and growth of a temperature-sensitive leucyl-
tRNA synthetase mutant at nonpermissive temperatures. The leuV operon also exhibited the stringent response in multicopy plasmids. The promoters of all four leucyl operons were fused to the gene for
beta-galactosidase and inserted into the chromosome by using bacteriophage lambda. All except the leuX promoter displayed growth rate-dependent regulation, consistent with the recent report that the concentration of
tRNA(6Leu) actually decreases as growth rate increases. The leuV promoter fused to the
beta-galactosidase gene showed a decrease in efficiency in the presence of extrachromosomal copies of rRNA genes. All chromosomal
tRNA genes examined showed decreased transcriptional activity following a stringent response, but the leuX gene responded to a lesser extent (3-fold versus 10-fold or more) than the others. Primer extension analysis of this promoter showed little if any response to
serine hydroxamate treatment, suggesting that multiple levels of control may exist or that promoter context effects are important in regulation.