The Regulatory Mechanisms Of DnaA-Mediated Gene Expression In Escherichia Coli

Escherichia coli is a prokaryotic model organism, containing two independent cellular processes of DNA replication and cell divitsion. However, the regulatory mechanism of coordination between these two processes and with gene expression has been elusive. The DnaA protein is the DNA replication initiator, also a transcription factor, In this study, we have revealed regulatory mechanisms for gene expression, which is mediated by DnaA. We found that the transcription of the uvrB gene increased 3.5-fold in the AlexAAsulA mutant and dnaA345 mutant, the increase was doubled in the △lexA△sulAdnaA345 mutant. The uvrB gene has 3 promoters, namly uvrBp1, uvrBp2 and uvrBp3. The uvrBp1-2 promoter was found to play an essential role in control of uvrB gene expression and be repressed by both DnaA and LexA. A DNase I footprinting assay showed that LexAbound tightly to LexA-boxl, but bound to LexA-box2,3 and 4 with lower affinity. However, LexA was found to be peeled off LexA-box2 and 3 in a manner of DnaA-concentration-dependent, but not from LexA-boxl. The results suggest that DnaA and LexA compete for binding to the uvrBp3 promoter since DnaA-box6 overlaps with LexA-box4. LexA-boxl was important for control of uvrBp1-2 transcription while repression of the uvrBp3 promoter was largely dependent on the DnaA-box6. Moreover, two other SOS genes, recN and dinJ, were also found to be repressed by both DnaA and LexA. Interestingly, A simultaneous absences of DnaA-and LexA-repression led to elongated cells with incomplete replication, aberrant nucleoid, and slow growth. Further analysis showed that DnaA-box overlapped with LexA-box in several SOS promoters in some gram-negative bacteria. The results allow us to propose a model for maintenance of the genome integrity through regulation of the SOS response by DnaA and LexA.In addition, the expression of his operon was found to be reduced in DnaA-overexpressing cells. Deletion of hisL and the stem-loop structure resulted in constitutive expression of the his operon and elimination of DnaA-repression on the his operon, suggesting that DnaA-repression depends on the stem-loop structure in the his operon.Taken all data described above together, we propose that DnaA coordinates DNA replication with the SOS-mediated DNA repair and histidine metabolism.