Cloning of a gene localized and expressed at the ecdysteroid regulated puff 74EF in salivary glands of Drosophila larvae.

The puffing cycle of salivary gland chromosomes of Drosophila larvae, which initiates the developmental path to pupariation, is induced by ecdysteroid hormone. Its action leads to prominent puffs at loci 2B5, 74EF and 75B. Fragments of the 74EF puff of the D. melanogaster 3L chromosome were microdissected from salivary gland squashes. EcoRI-digested DNA of these fragments was cloned into lambda phage. Clones were screened with puff stage-specific cDNA probes. Thirteen out of 650 clones hybridized preferentially with puff stage 4-specific cDNA. The prominent early puffs at 74EF and 75B are most active between puff stage 4 and 6. Therefore, one of the 13 lambda phages was chosen for further analysis. It was used to isolate 24 kb of Drosophila DNA from genomic libraries. The DNA hybridized in situ to locus 74F. The 74F DNA coded for a transcript, which was made in salivary glands, but not in fat body of third instar larvae. It accumulated in K(C) cells in response to ecdysteroid treatment. The polyadenylated transcript size was 2.7 kb as judged by Nothern blot analysis. The transcription start site of the 74F gene has been mapped. Sequences upstream of the transcription site contain several sequence elements common to other eucaryotic genes, including potential Z-DNA forming sequences. Also, there is sequence homology to upstream sequences, which have been involved in the regulation of transcription of the salivary gland glue protein 4 gene.