SOS Response


Bacteria have developed an SOS response to mend DNA damage caused by deleterious agents such as radiations and chemicals. Mutagenesis, one of the SOS functions, has permitted bacteria to adapt to short‐ and long‐term adverse living conditions.

Keywords: SOS signal; RecA nucleoprotein filament; SOS induction; LexA repressor; SOS mutagenesis

Figure 1.

From DNA lesions to SOS functions. (1) Damage to DNA, (2) nature of SOS‐inducing signal, (3) inactivation of repressors and mutagenesis proteins, (4) induced molecular reaction, (5) expressed SOS genes, and (6) SOS‐induced functions. E, early; D, delayed.

Figure 2.

Regulation of SOS genes. (a) In steady state, the replication fork proceeds normally. The SOS genes, four of which are represented with LexA repressor bound to their operator, express small amount of protein, except for the tightly repressed umuDC operon. (b) Following DNA damage and the appearance of SOS‐inducing signals, early LexA cleavage occurs, catalysed by RecA nucleofilament on a blocked replication fork. SOS genes are expressed, among which are DNA repair genes. When mutagenesis proteins (UmuDC proteins) are produced, UmuD is processed in the presence of RecA nucleofilament, leading to mutagenesis. When DNA is repaired, SOS signals disappear and the cell reverts to steady state.

Figure 3.

Induction of λ phage genes by λ cI repressor. (a) In the steady state, λ phage genes are repressed by phage cI repressor. (b) Following DNA damage and a blocked replication fork, RecA nucleofilament catalyses λ repressor cleavage, leading to phage production.

Figure 4.

Time of gene expression during the cell cycle. Gene inducibility measured as the strength of LexA binding to the SOS operators–promoters is represented as the ordinate. In the abscissa, the operators–promoters of the SOS genes are aligned, following sequential induction during the cell cycle.

Figure 5.

Molecular mechanism of SOS mutagenesis.



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Further Reading

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Defais, Martine, and Devoret, Raymond(Sep 2005) SOS Response. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1038/npg.els.0003874]