Target gene responses differ when transcription factor levels are acutely decreased by nuclear export versus degradation.

Defining the time of action for morphogens requires tools capable of temporally controlled perturbations. To study how the transcription factor Dorsal affects patterning of the embryonic dorsal-ventral axis, we used two light-inducible tags that result in either nuclear export or degradation of Dorsal when exposed to blue light. Nuclear export of Dorsal results in loss of expression for the high threshold, ventrally-expressed target gene () but retention of the low threshold, laterally-expressed target gene (). In contrast, degradation of Dorsal results in retention of loss of , and lower nuclear levels than when Dorsal is exported from the nucleus. To elucidate how nuclear export results in loss of but degradation does not, we investigated Dorsal kinetics using photobleaching and found it reenters the nucleus even under conditions of blue-light when export is favored. The associated kinetics of being imported and exported continuously are likely responsible for loss of but, alternatively, can support . Collectively, our results show that this dynamic patterning process is influenced by both Dorsal concentration and nuclear retention.