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A new paper from postdoctoral fellow Stephan Uphoff in the Biochemistry department has revealed that random variation in the DNA repair capacity of cells can lead to genetic variation.

E. coli cells treated with DNA methylation damage induce the adaptive response by activating Ada protein expression. The microscopy image shows fluorescently tagged Ada in yellow. Despite identical genetic makeup and treatment, a fraction of cells fails to induce the Ada response (in grey). Scale bar: 5 m (Click to Enlarge)

The results are published in Science (1) and are the fruition of a collaborative project between Dr Uphoff in Professor David Sherratt's lab and the lab of Professor Johan Paulsson at Harvard Medical School. They provide insight into how phenotypic variation can lead to genetic variation - a new twist on studies exploring the impact of variability in gene expression between cells.

A physicist by training, Dr Uphoff has spent the last few years developing and applying live cell imaging techniques. Currently funded by a Sir Henry Wellcome Postdoctoral Fellowship from the Wellcome Trust and a Junior Research Fellowship at St John's College in Oxford, he has been using single-molecule imaging to study mechanisms of DNA repair in bacteria, in both the Sherratt and Paulsson labs.

The newly published study explores the consequences of heterogeneity in a bacterial DNA repair process. Whilst there has been lots of discussion about noise in gene expression giving rise to phenotypic heterogeneity in genetically identical cells, there have been few studies that go beyond transient variations in gene expression. In the case of DNA repair, however, any transient heterogeneity could persist over long timescales in the form of mutations.

In the bacterium Escherichia.coli, the adaptive response protects cells against the toxic and mutagenic effects of DNA methylation damage. This requires Ada protein, which as well as directly repairing methylated DNA, also activates ada gene expression. It does this via a positive feedback mechanism - ada expression is increased a thousand-fold by methylated Ada which acts as a transcriptional activator after transfer of a methyl group from damaged DNA onto the protein during the repair process.

Another feature of the DNA damage response is that Ada protein is present in low numbers in cells before DNA damage. 'We hypothesised that there should be substantial heterogeneity in the adaptive response between cells because positive feedback tends to amplify the noise that is inherent in low molecule numbers,' says Dr Uphoff.

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Department of Biochemistry, University of Oxford