Dense cluster of genes is located at the ecdysone-regulated 3C puff of Drosophila melanogaster.

The 3C11-12 polytene bands of the Drosophila melanogaster X chromosome give rise to a prominent puff, whose regression is triggered by the increase in the titre of the steroid hormone 20-hydroxyecdysone occurring before the metamorphosis. Here, we report the molecular characterization of three genes, named ng-2, ng-3 and ng-4, which we found to be closely linked to each other and to Sgs-4, Pig-1 and ng-1, three other genes previously mapped at this polytene region. All six genes are, in fact, arranged in a tightly linked cluster spanning a DNA segment of only 11 kb. With the exception of ng-4, all the clustered genes are highly expressed only during the larval life and share the same tissue-specificity, being mainly transcribed within the salivary glands. In addition, two members of the cluster, ng-1 and ng-2, show a very high degree of sequence homology, clearly indicating that they are related to each other by means of a duplication event. Interestingly to note, the entire cluster shows a peculiar genomic location, extending across two introns of the memory gene dunce, a large gene of Drosophila whose organization has proved to be remarkably complex.