DNA is important stuff. It’s present in all living organisms on the planet (or ‘almost all’ if you wish to remain friends with virologists) and contains the information required to produce and organise the proteins within a cell. If the DNA is damaged, the cell will very quickly find itself in danger. In multicellular organisms the result of one cell going haywire is so catastrophic that the usual response to anything other than very simple DNA damage is for the damaged cell to commit honourable suicide.
In unicellular bacteria, the results of DNA damage only affect the cell in question, and as the internal organisation of the cell is more flexible the cell can get away with slightly more shoddy DNA repair. In some cases this can even be an advantage; changes to the DNA may result in new genes or gene combinations which may be of benefit to the bacteria.
The bacterial response to DNA damage is known as the SOS response. There are two main proteins involved – one, LexA, to keep the response switched off while the cell is healthy and the other, RecA, to turn it on when DNA damage occurs. In normal healthy cells the LexA protein binds to a certain section of the bacterial DNA called the ‘SOS box’ which codes for over 50 genes while RecA floats around the cell looking for damaged DNA. If it finds any, it binds to it and stimulates the breaking of the LexA protein. The SOS box genes are therefore released and the proteins that deal with DNA damage can be made.