The Drosophila ribosomal protein S6 gene includes a 3' triplication that arose by unequal crossing-over

Ribosomal protein S6 (rpS6) is the major phosphoprotein of the small ribosomal subunit of eukaryotes and is phosphorylated in response to treatment with mitogens and other stimuli. We have examined the organization of the rpS6 gene of Drosophila melanogaster. Comparisons of a cDNA with genomic DNA identify a transcription unit including three exons. Two tandem repeats downstream of this transcription unit reiterate divergent copies of the third exon and flanking regions. Comparisons of these three repeats with respect to nucleotide base substitutions and deletions or insertions show clearly that they arose via a duplication and subsequent crossing-over between misaligned copies. Although no direct evidence exists that the downstream exons are transcribed, the maintenance of open reading frames in spite of extensive genetic changes is consistent with a protein-coding function.