Uneven distribution of cloned transcribed DNA sequences in polytene chromosomes of Drosophila melanogaster.

DNA fragments actively transcribed at different developmental stages of Drosophila melanogaster were cloned. In situ hybridization was used to study their distribution in the polytene chromosomes. The clones were dot-hybridized to poly(A)+ RNA isolated at different developmental stages. Fifty-four of 67 clones were unique, 37 of them were localized. An irregular distribution of the cloned sequences was revealed. Twenty-three clones are localized in chromosome 3, six clones in the X-chromosome, and only eight in chromosome 2. There are transcriptionally active regions where, on a relatively small area of the chromosomes, a number of clones are localized (sections 18-19, 69-71, 82-85, 95-97). RNAs transcribed from the cloned sequences code for about 0.005 to 0.2% of the total poly(A)+ RNA.