It is now widely accepted that stress conditions lead to an increase in the frequencies of mutation measured by selection. The question now raised by the "Directed mutation" issue is the specificity of the changes induced by particular stress regimes. To answer this question, it is necessary to characterize all DNA changes regardless of whether they facilitate growth under selective conditions. This can now be done by PCR technology. PCR makes it possible to examine specific regions of the genome following stress and detect novel DNA structures, even if they can not be selected by growth. Initial results demonstrated that a specific class of DNA rearrangement mediated by the Mu phage transposable element did not measurably occur under normal growth conditions but were easily detectable at relatively high frequencies under selective conditions. Use of PCR will now allow detection of the frequency and time of occurance of such genetic rearrangements with and without the selective techniques that are usually required for measuring mutation rates.
