The gene of the purple non-sulfur bacterium no. 7 was isolated by a PCR-based method and sequenced. The complete nucleotide sequence consists of 1089 bp encoding a polypeptide of 363 amino acids which is most closely related to the RecA proteins from species of and A -deficient strain of no. 7 was obtained by gene replacement. As expected, this strain exhibited increased sensitivity to DNA-damaging agents. Transcriptional fusions of the promoter region to confirmed that the no. 7 gene is inducible by DNA damage. Primer extension analysis of mRNA located the gene transcriptional start. A sequential deletion of the fusion plasmid was used to delimit the promoter region of the gene. A gel mobility shift assay demonstrated that a DNA-protein complex is formed at this promoter region. This DNA-protein complex was not formed when protein extracts from cells treated with DNA-damaging agents were used, indicating that the binding protein is a repressor. Comparison of the minimal no. 7 promoter region with the promoter sequences from other α-Proteobacteria revealed the presence of the conserved sequence GAACA-N-G(A/T)AC. Site-directed mutations that changed this consensus sequence abolished the DNA-damage-mediated expression of the gene, confirming that this sequence is the SOS box of and probably plays the same role in other α-Proteobacteria.


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