Transcription and metabolism of RNA from the Drosophila melanogaster heat shock puff site 93D

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Abstract
Characteristics of the major heat shock puff site 93 D and the RNA transcribed from it have been investigated by hybridization to polytene chromosome preparations and to recombinant DNA. By saturation in situ hybridization, the length of the transcribed region at 93 D is twice that of the mRNA coding region at the heat shock puff site 87 A. From the known length of the heat shock mRNA sequence at 87 A, we calculate that the minimum length of the transcribed region at 93 D is 9.6 kb (kb = kilobase, i.e., 1,000 nucleotides). — The metabolism of RNA transcribed from 93 D has been compared with that of RNA coding for the major heat shock protein hsp70 in cells incubated for one hour at 35° C. Hsp70 mRNA sequences, assayed by hybridization to a specific recombinant DNA probe and by in situ hybridization to 87 A, were found in both poly(A)+ and poly(A)− cytoplasmic RNA and were more concentrated in cytoplasmic RNA than in nuclear RNA. In contrast, sequences complementary to 93 D, assayed by in situ hybridization, were more concentrated in nuclear than in cytoplasmic RNA. This implies that sequences from 93 D exit from the nucleus at a lower rate and/or are turned over in the cytoplasm at a higher rate, than sequences from 87 A. Site 93 D is also unusual in that its transcribed region is represented in both poly (A)+ and poly (A)− nuclear RNA, even though 93 D-complementary RNA in the cytoplasm is predominantly poly (A)−. Finally, only 28–58% of the length of DNA transcribed at 93 D is represented in cytoplasmic RNA, indicating that only a portion of the sequences transcribed from 93 D are exported from the nucleus. The transcripts from two heat shock loci, 93 D and 87 A, thus appear to be metabolized in significantly different ways.