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

Abstract

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