Dual level control of the Escherichia coli pheST-himA operon expression. tRNA(Phe)-dependent attenuation and transcriptional operator-repressor control by himA and the SOS network.

Previous studies of phenylalanyl-tRNA synthetase expression in Escherichia coli have established that the pheST operon transcription is controlled by a Phe-tRNA(Phe)-mediated attenuation mechanism. More recently, the himA gene, encoding the alpha-subunit of integration host factor, was recognized immediately downstream from pheT, possibly forming part of the same transcriptional unit. By using the in-vitro transcription and S1 mapping techniques, transcription termination after pheT could be excluded, indicating that himA can be expressed from polycistronic messenger RNAs encompassing the pheST region. However, the presence of a secondary promoter able to express himA and located within pheT is demonstrated. To further investigate the regulation of the pheST-himA operon expression, genetic fusions between various parts of this operon and the lacZ gene were constructed and studied. Our results confirm the autoregulation of himA previously described, and demonstrate that it occurs through the modulation of the secondary promoter activity within pheT. Surprisingly, it is found that the pheST promoter is also submitted to the same control. Consistent with this, DNA sequences homologous to the integration host factor binding site consensus are present at the level of both promoters. However, evidence in favor of two different repressor complexes is provided. Previously observed SOS induction of the himA expression is shown to occur through the modulation of both promoter activities. Contrasting with the other genes under SOS control, the LexA protein binding site consensus sequence could not be found in the two promoter regions. This suggests that either the LexA protein directly participates in the formation of an active holorepressor, or that the product of an SOS gene is able to inhibit the formation or the binding of such a repressor. Finally, our results indicate that the pheST-himA operon expression is controlled by two different mechanisms acting independently. (1) The phenylalanyl-tRNA synthetase and the himA product expressions are controlled by an operator-repressor type mechanism, in which the himA product and the SOS network are involved. (2) Through its partial cotranscription with pheST, himA expression is also under attenuation control. The latter control may provide a way to couple the intracellular concentration of the himA product to the functional state of the translational apparatus.