Question

Regarding the SOS repair response:

a.   Explain what the SOS response is, indicate under what circumstances it is active, and what it achieves.

b.   Explain the role of the Lex A and Rec A proteins in the SOS response.

Answer 1

a.The SOS response ( Save Our Soul) is a global response to DNA damage in which the cell cycle is arrested and DNA repair and mutagenesis is induced. The system involves the RecA protein (Rad51 in eukaryotes). The RecA protein, stimulated by single-stranded DNA, is involved in the inactivation of the repressor (LexA) of SOS response genes and hence inducing the response. It is a repair system that contributes significantly to DNA changes observed in a wide range of species.

b. During normal growth, the SOS genes are negatively regulated by LexA repressor protein dimers. Under such normal conditions, LexA binds to a 20-bp consensus sequence, known as SOS box in the operator region for those genes. Activation of the SOS genes occurs after DNA damage by the accumulation of single stranded (ssDNA) regions generated at replication forks, where DNA polymerase is blocked. RecA makes a filament around these ssDNA regions which consumed ATP, and becomes activated. The activated form of RecA interacts with the LexA repressor to facilitate the LexA repressor's self-cleavage from the operator.Once the pool of LexA decreases, repression of the SOS genes lowers down according to the level of LexA affinity for the SOS boxes. Operators that bind LexA weakly are the first to be fully expressed. Through this LexA can sequentially activate different mechanisms of repair. Genes having a weak SOSbox (such as lexA, recA, uvrA, uvrB, and uvrD) are fully induced in response to even weak SOS-inducing treatments.