Activation of silent transposable elements.

It is well known among maize geneticists that agents that cause chromosome breakage can activate quiescent transposable elements. However, other than temporarily relieving position effect, it is difficult to understand how these events can lead directly to activation. One possibility is that chromosome breakage can initiate a process in the cell resulting in a higher rate of spontaneous mutation. Such a system could be analogous to the SOS response of Escherichia coli in which an error-prone repair system is induced. Chemical mutagens that cause little chromosome breakage but add bulky adducts to the DNA can induce the SOS response. In seed homozygous for a1-m2(8004), wx-m8, no active Spm, that had been treated with ethyl methanesulfonate, we observed activation of Spm at the rate of 1.1 x 10(-4). The spontaneous rate of activation in this material was 1.2 x 10(-5). Most of the activation events occurred as single kernels. This result contrasts with sectors covering at least one-eighth of the ear that would have been expected if activation had occurred as a direct result of mutagenesis in the mature kernel. The late timing of these events suggests that the activation, in most instances, may not be the direct result of chemical mutagenesis.