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Analysis of the Drosophila rDNA promoter by transient expression.

We have examined the expression of the bacterial gene chloramphenicol acetyl transferase (CAT) under the control of the Drosophila rDNA promoter following transfection into Drosophila tissue culture cells. Constructs having an entire NTS, corresponding to approximately 3640 base pairs of upstream rDNA sequence, or constructs with 306 base pairs of upstream sequence respectively, are transcribed at 5 fold or 2 fold higher levels than a construct with 43 base pairs of upstream DNA. In co-transfection experiments, the construct with the entire NTS competes for transcription 20 fold more effectively than the construct with 306 base pairs of upstream sequence. Constructs having either 72 base pairs or 60 base pairs of upstream rDNA sequences, on the other hand, are transcribed very much less efficiently than constructs with either 306 bp or with only 43 bp of upstream DNA. These sequences, which reduce levels of rDNA transcription in the absence of additional upstream DNA, lie in a region in which the rDNA promoter differs from its duplications within the NTS.