The origin recognition complex (ORC) is a highly conserved complex found in all eukaryotes.  It is composed of six proteins with a mass of 400 kD.  ORC binds to the yeast A and B1 elements on the A-T–rich strand and is associated with ARS elements throughout the cell cycle.

TLS Online TPP Program

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#Unit 3. Fundamental Processes

Damage leads to the proteolytic destruction of a transcriptional repressor (the LexA repressor) that controls expression of genes involved in the SOS response, including those for DinB, UmuC, and UmuD, the inactive precursor for UmuD0.

TLS Online TPP Program

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#Unit 3. Fundamental Processes

Cleavage of both LexA and UmuD is stimulated by a protein called RecA, which is activated by single-stranded DNA resulting from DNA damage.

TLS Online TPP Program

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#Unit 3. Fundamental Processes

LexA repressor (Mr 22,700) inhibits transcription of all the SOS genes, and induction of the SOS response requires removal of LexA

TLS Online TPP Program

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#Unit 3. Fundamental Processes

The LexA repressor is inactivated when it catalyzes its own cleavage at a specific Ala–Gly peptide bond.

TLS Online TPP Program

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#Unit 3. Fundamental Processes

SOS response in E. coli: 

The LexA protein is the repressor in this system, which has an operator site near each gene. Because the RecA gene is not entirely repressed by the LexA repressor, the normal cell contains about 1,000 RecA monomers. 

TLS Online TPP Program

#Id: 3412

#Unit 3. Fundamental Processes

SOS response in E. coli:

When DNA is extensively damaged (such as by UV light), DNA replication is halted and the number of single-strand gaps in the DNA increases.

RecA protein binds to this damaged, single-stranded DNA, activating the protein’s co-protease activity.

While bound to DNA, the RecA protein facilitates cleavage and inactivation of the LexA repressor. When the repressor is inactivated, the SOS genes, including RecA, are induced;