A Drosophila rRNA gene located in euchromatin is active in transcription and nucleolus formation.

P-element transformants of a single rRNA gene (rDNA) were used to investigate the relationship between the organization of the nucleolus organizer (NO) and rDNA function in Drosophila melanogaster. In situ hybridization to rRNA in polytene nuclei of salivary glands demonstrated that an rRNA gene can be transcribed at a high rate when inserted into chromosomal sites other than the NO. Structures that resemble morphologically the endogenous nucleoli ('mininucleoli') were associated with four different euchromatic sites of rDNA insertion. Molecular analyses revealed that these mininucleoli contained both rRNA and an antigen specific to nucleoli. Phenotypes resulting from rDNA deficiencies were rescued partially by the presence of the transformed rDNA, indicating that the transcripts and mininucleoli associated with the rDNA insertion sites were functional. Thus, two conserved features of rDNA organization in eukaryotes, namely tandem repetition and heterochromatic localization, are not required for rRNA gene function. We conclude that 'nucleolar organizing activity' is an intrinsic property of the rDNA or its RNA products.