DNA Repair and Mutagenesis

Main sites of modification of DNA bases by methylating agents.

The arrow sizes reflect the relative yields of the different methylated products. Atoms or groups involved in Watson–Crick hydrogen bonding are shown in red. Bases methylated at positions indicated by the black arrows are processed primarily by AlkA; those methylated at sites marked with blue arrows are addressed by O6-methylguanine DNA methyltransferase, and those methylated at positions marked with the green arrows are processed by AlkB

AlkB-mediated demethylation of N3-methyladenine and N1-methylcytosine.

The methyl groups are converted to hydroxymethyl moieties, which are spontaneously lost in the form of formaldehyde to regenerate the unmodified bases (the aberrant methyl groups and their oxidation products are shown in red). Note that the cofactors (blue) are consumed during the reaction in stoichiometric amount

WT uvrA uvrB uvrCUV dose J/m2

05

10204080

120

E. coli that are unable to remove DNA lesions are sensitive to UV irradiation

People that are unable to remove DNA lesions are also sensitive to UV irradiation

Photo taken from Friedberg, EC., Walker GC., Siede, W (1995) in DNA Repair and Mutagenesis (ASM Press, Washington DC)

OFFOFFOFFOFFONONONONumuCuvrArecAlexAOFFOFFOFFOFFumuCuvrArecAlexAOFFOFFOFFOFFumuCuvrArecAlexAumuCuvrArecAlexAumuCuvrArecAOFFOFFOFFOFFlexALexA normally binds to the promoter regions and represses more than 40 genes

However, following UV-induced DNA damage or other replication arresting events...

...RecA binds to the single strand regions that are generated when replication forks are arrested

The bound RecA serves to protect the replication fork DNA from degradation and also promotes the autocatalyic cleavage and subsequent degradation of the LexA protein, leading to the derepression of the SOS genes.

Once the arresting lesions have been repaired and replication has been restored, the single strand substrate for RecA binding is no longer present, LexA is no longer cleaved and its cellular concentration accumulates. Then, once again, LexA binds to the promoters and represses the expression of the SOS genes, including that of lexA and recA.

=RecA=LexA=UV-induced DNA damage DNA replication bubble

0.1

1

10

oraA

0 20 40 60minutes

recA

0 20 40 60minutes

lexA-77

2820 kb

-UV

lexA1

+UV

wildtype