Differential responses of the dopa decarboxylase gene to 20-OH-ecdysone in Drosophila melanogaster

The dopa decarboxylase gene ( Ddc) of Drosophila melanogaster responds to 20-OH-ecdysone in the mature larval epidermis and in imaginal discs (presumptive adult epidermis) in a tissue-specific manner. Exposure of the mature larval epidermis to 20-OH-ecdysone caused a rapid accumulation of DDC transcripts. In the absence of protein synthesis, transcript accumulation was substantially reduced suggesting an indirect hormonal effect on DDC transcription (and/or RNA turnover). By contrast, neither DDC activity induction nor transcript accumulation was detected in imaginal discs cultured in the continuous presence of the hormone. However, when discs were exposed to 20-OH-ecdysone and then cultured in its absence, DDC activity and DDC transcript levels started to increase 6 hr after hormone withdrawal. A Northern analysis failed to reveal any novel transcripts in discs making the utilization of an alternative promotor an unlikely explanation for the very different responses of the Ddc gene in the two epidermal tissues. The results demonstrate that the Ddc gene in the larval epidermis responds rapidly to an increase in hormone titer. In imaginal discs a fall in hormone titer is required before DDC transcripts accumulate.