If there is one protein with little chance of leading a private life, it's p53. This tumour suppressor and its encoding gene have been thoroughly tinkered with over the years, with the result that we have a detailed molecular picture of how this so-called 'guardian of the genome' protects cells from genome damage by either promoting DNA repair or cell death. Galit Lahav and colleagues have now witnessed p53 at work in individual living cells. By fluorescently labelling human p53 and its regulator MDM2, they show that discrete p53 protein levels are activated in discrete quantities when DNA is damaged, and that the number of these 'pulses' depends on the severity of the damage.

The negative-feedback relationship between p53 and its partner MDM2 is well known: DNA damage lowers MDM2 levels, which in turn stabilize the p53 protein so that it can attempt to repair the damage. More p53 also means higher MDM2 transcription and thereby p53 destabilization. To visualize these dynamics, the authors used time-lapse fluorescent microscopy on living human cells that expressed MDM2–YFP and p53–CFP fusion proteins, which glow yellow and cyan, respectively. The cells were zapped with gamma rays, which breaks DNA, and the levels of each protein were examined every 20 minutes for 16 hours. Fluorescence imaging is carried out routinely, but what is new here is the ability to look at individual cells, as the average signal released by a pool of cells would be impossible to resolve.

What the authors expected to see from this experiment was an 'analogue' behaviour, in which the strength of the output of the system matches the input — that is, where the amount of p53 protein increases in a graded manner with the severity of DNA damage. Instead, when the cells were irradiated, the two components of the p53–MDM2 feedback loop were activated in a series of discrete bursts, each containing a fixed amount of protein. The average height and duration of each pulse remained unchanged even when the DNA was more badly damaged; instead, the cells responded by increasing the number of pulses.

This behaviour — which is described as 'digital' because the magnitude of the input is translated into a number of discrete outputs — is important in some biological systems, such as spiking neurons, but defies theoretical expectations of how a negative-feedback relationship should operate. It's a trickly problem to address, but the authors speculate that the gradual increase in p53 protein that is afforded by repeated pulses is a failsafe mechanism that prevents the downstream repair enzymes from swamping the cell and triggering its death.